Drawing craft patterns on your computer is easier than you think with Inkscape, a powerful, free, open source vector graphics editor.
First, you will need to download and install the program: Inkscape's Download Page.
To illustrate the basic process, we will be making a simple square.
Open the program. Select File->Document Properties (Shift-Control-D). Select the appropriate page size, usually US Letter. Change your default units to in for inches, or if you prefer metric cm.
Select the Square Tool (F4) from the tool bar that runs vertically on the left of the application. To draw a square: click on the white canvas in the center. Keep holding the mouse button down as you drag off to one side. When you are done dragging, release the mouse button. Hold down the control key while dragging to constrain the shape to a perfect square.
The following steps require the square you made to remain selected. If you should accidentally deselect the square, simply select the Arrow Tool (F1) from the top of the left tool bar and click on the square you drew to reselect it.
If you need to zoom in or out as you work, press the - or + keys. Or: hold down the control button on your keyboard while scrolling the wheel on your mouse. Or use the Zoom Tool (f3) to draw a box around the area you want to magnify. Numbers 1-6 on your keyboard will show you different zoom levels as well.
Along the bottom of the application is a row of color chips. Click on any chip to change the color of the square. For printable patterns, light colors are best.
Directly under the color chips, on the left of the screen, you will see the words Fill and Stroke with color bands next to them. These indicate the color of the fill and the outline(stroke) of the currently active shape. Right click on the color band next to the word "stroke" and select black. To select a different color, hold down the shift key and click on the desired color-chip.
The number to the right of the stroke color band represents the thickness of the stroke. Right click on it and select a different width to see how it looks.
If you do a regular click (rather than a right-click) on the stroke color, you will bring up the Fill & Stroke dialogue (Shift-Control-F). This dialogue is an alternate way to select a color for either the fill or stroke, as well as other properties. On the stroke paint tab, use the sliders to change the color. Make sure the solid blue box at the top is selected for a visible stroke. To make the stroke invisible, select the "X".
The stroke style tab will allow you to change the thickness. You can also select a dotted line using the dashes entry.
In bottom row of the tool bar that runs along the top of the application, you will see a section indicating the width and height (marked as W and H) of the selected item. If the measurement is in "px" be sure to change the drop-down to "in" for inches or "cm" for centimeters. Enter 3 (or whatever number you want) into both the height and width. The size of your square will change. Clicking the little lock between height and width will make the two numbers change in tandem.
You can also re-size by clicking and drag on the corners of the bounding box that appears on the drawing canvas when you select the drawn object with the arrow tool. A selected item can be re-sized using the keyboard by pressing the < and > keys. Hold down the control key while you do so to make the resize work faster.
Use File->Print (control-P) to print your work. You should get a square that measures exactly 3 inches (or whatever size you entered).
Save your work (control-S). Inkscape saves your master files in *.svg format. Computers without inkscape installed will not be able to open these files.
To create a version that can be opened universally, used on websites, etc, you will need to export your drawing. Once the image is exported, it can no longer be edited, so it is important to keep your *.svg file even if you have an exported copy of your drawing saved. Use File->Export Bitmap (shift-Control-E) to export.
For printing, you will want to use between 200-300 dpi. For screen viewing only, use 72 dpi. Use the "browse" button next to the filename field to name and locate your file. From the top of the dialogue, select "page" if you want the entire document, "drawing" if you want only the area with drawings on it, "selection" if you want only the selected part of the drawing.
This will produce a file of type *.png. Some older computers and some applications may have problems with this file type, so you may want to open it in a program like Photoshop or Irfanview (free download) and save as a *.jpg.
The Square Tool produces rectangles as well as squares. Below the
square tool is the Circle Tool for circles and ovals and the Star
Tool for stars and closed shapes like hexagons, pentagons, etc.
The shape that the Star Tool creates is controlled by the input boxes that appear on the bottom row of the top tool bar when the star is selected. Draw a star (click, drag and release, just like drawing a square or circle) and then enter different values to change the appearance of the shape you created. Some examples are shown below.
To make your star/polygon a certain size, select it using the Arrow Tool (F1) to make the top tool bar display the size information. You may need to rotate the object, as the height and width are calculated from the bounding square for irregularly shaped object. To rotate: click twice on the object with the Arrow Tool selected, causing little rounded arrows to show up on the corners of the bounding box:
Click and drag on one of the corner arrows to spin the shape. Alternately, use the  keys on your keyboard. Holding down "alt" while you do so will reduce the increments of the turns for fine-tuning.
If you need to measure part of a drawing, select the Pen Tool from the left tool bar. Click-release where you want to start measuring and then move the mouse to the desired end point without clicking. A red line will appear.
At the bottom status bar, below the color chips, you will see information about the line you are drawing, including its length (in this case 1.16"). Press esc on your keyboard to cancel the line when you are done measuring.
One way to make a custom shape is to start with built-in shapes and modify them.
Draw a square. Select Object to Path from the Path menu item at the top of the application.
Select the Node Tool (f2) from the left, right below the arrow tool. Click on the square you drew. Little gray square boxes (nodes) will appear at the corners.
Click and drag the corner nodes to move them (they will turn blue when selected), or on the sides of your square to bend the lines between them. Once a line is bent, you will see blue lines ending with circles (handles) coming out of the node. You can click and drag on the handles to change the shape of the adjacent curve.
With a corner node selected, click on one of the node-modifying buttons on the bottom row of the top tool bar. The auto-smooth button (marked in red on the screenshot below) was used to produce this rounded corner:
To add new segments: click on one of the sides of your shape (between the nodes) to select that segment.
Click on the Add Node tool at the far left of the top control bar to add a new node in the middle. The new node plus the two nodes on the end of the segment will be selected, so click off the shape to deselect them, then click on the new node (shown in red below) to select just it. Drag it to a new position.
You can also just double-click the line where you want the new node to appear.
If you want a half-square or half-rectangle triangle, (A) first draw the square or rectangle with the Square Tool and select Object to Path from the Path menu item at the top of the application. (B)Select the Node Tool (f2) and click on one of the corner nodes to select it. Press "delete" on your keyboard to remove it (C) Control-click on the end of the handles adjacent to the curved edges to straighten them out, OR select the nodes and use shift-C to turn them all into cusp/corner nodes.
If you want to make an equalateral triangle, (A) draw a square and run Object to Path from the Path Menu. Select the Node Tool (f2) and use it to select one side of the square. Click the "add node" button from the bottom row of the top menu bar. (B) Select & delete the two adjacent nodes.(C&D) Turn all the remaining nodes to cusp/corner nodes with shift-C (or by control-clicking on their handles).
Rather than starting with a built-in shape, you can draw your shape free form using the pencil or pen tool. Then use the Node Tool to alter your line just as you did with the converted square above. For pattern making, the pen tool is recommended over the pencil tool.
You can also construct your pattern piece by combining simple shapes:
In this example, two overlapping circles were drawn. Both were selected (using the Arrow Tool, either click off to one side and draw a bounding box over both shapes OR use control-A to select all shapes OR shift-click on each shape to add to the selection) and then Path->Union was selected from the top navigation menu.
If you choose Path->Difference you get this:
If you choose Path->Intersection you get this:
You can also create half of your desired shape (A), then copy/paste to create a duplicate (or control-D), press the H key to flip the shape horizontally (B), move it into place atop the original (C) select both and the run Path->Union to combine them into one shape (D).
If you own a scanner, you also have the option of drawing your pattern by hand and then importing it into Inkscape. Select File->Import from the top menu and resize/move your image to the canvas if necessary.
First, check to see if Inkscape's automatic tracing will work with your drawing. The cleaner the drawing and the whiter the background, the more likely this is. Select Path->Trace Bitmap from the top menu.
Experiment with the settings, then click "update" until the preview looks reasonable. Click OK when done.
Your automatically generated drawing will be atop your imported image, so drag your image off to one side.
In this example, the drawing was too messy and full of extra lines for a good tracing:
Even after running Path->Simplify, it's still not clean enough to use for a pattern.
Instead, manually draw your pattern over the scan, using it as a guide only. To make it easier to see your new drawing, select the scan and reduce its opacity (marked "O") to 30-40% via the lower left tool bar (or Shift-control-F, adjust the opacity slider).
Select the Pen Tool from the left tool bar (next to Fill/Stroke). Click around the outline of your drawn shape:
Switch to the Node Tool and adjust your lines into curves where necessary.
Work your way around the drawing, adding or removing nodes as neccesary to make your new line match the underlying scan.
Add all desired details until you are done:
Select & delete your original scanned drawing to leave only your inkscape work behind:
To create a pattern piece with a seam allowance, first create the shape without the seam allowance.
With the shape selected, select Edit->Copy (control-C), Edit->Paste In Place (control-Alt-V) from the top menu. Or use Edit->Duplicate (control-D).
The new copy will be on top of the old one, so use the End Key on your keyboard (or Object->Lower to Bottom from the top menu) to move it underneath.
Run Path->Outset ('Control-)') from the top menu bar.
You may need to run the outset command several times to get a seam allowance of an appropriate width. Use the Pen Tool to check the width (see "other shapes" above).
Notice that the outset shape is NOT just a bigger version of the original. It also changes shape were necessary: for example the segment above the arms comes to a point above the arms, rather than a short arc.
Optional: adjust the properties of the stroke of the outset shape and the original shape using the Fill & Stroke control palette. (shift-control-F).
To add text to your pattern piece, select the Text Tool (f8) from the left-tool bar. It is marked with an "A". With the text tool active, the bottom row of the upper tool bar allows you to set text parameters, including font, font size, and alignment.
Color is set by clicking on the color chips at the bottom of the page. Text may be selected, moved, rotated & resized using the Arrow Tool (f1).
Pattern matching dots can be created with the circle tool, and other marks drawn with the pen tool.
Use the pen tool also to draw alternation lines, fold lines, mark widths, etc. Using different colors or stroke dash patterns can be helpful.
Inkscape allows you to draw over the borders of the page when creating your pattern. But when you want to print, you will need to divide your pattern piece into sub-pieces that can be printed on one sheet of paper, cut out and taped together.
First draw your pattern and size it as desired, regardless of whether it will fit on a sheet or not. Select all the pieces and group them together with Object->Group from the top menu (control-G ).
Draw a temporary rectangle of size 7.5x10 inches over part of your pattern piece. This is a safe printable size on US letter paper, and will give you a little extra room to add notes around the edges if needed. Reduce the opacity of the rectangle (Shift-control-F, or lower left corner) if desired to be able to see your piece underneath.
Select both the rectangle and your drawing (ie, control-A) and then select Object->Clip->Set from the top menu.
Only the part of the drawing covered by the temporary rectangle remains visible. Select the new partial pattern piece, copy and paste it to a new document.
Now go back to your original document. Select the partial pattern piece again, and now unset the clipping: Object->Clip->Release. Move the mouse over onto the vertical ruler on the left side of the application. Click in the gray area, and then drag to the right. A red guide line will follow your mouse. Drag the line until it reaches the far side of your temporary rectangle. It will turn blue when you release the mouse-button:
Now use the Arrow Tool to move your temporary rectangle to the other side of the guide line.
Repeat the process of selecting the two objects, set the clipping path with Object->Clip->Set, copy to a new file.
If your pattern piece is bigger than 2 sheets of paper, you will need to continue this process, moving the temporary rectangle along until you have masked out each desired section into its own file. Horizontal guide-lines can be used as well if the piece is tall as well as long.
As a finishing touch, you may want to draw a lip along the edge that will need to be joined and add some text instructing the end-user where it needs to be attached.