Grab a pair of old jeans from the depths of your closet or a thrift store and make yourself a new pair of skinny jeans. This method doesn't require another pair to act as a model or a second person to help you pin and ensures that you will be able to get your new jeans over your feet without having to add zippers.
Select a pair of jeans that fits you snuggly through hips and thighs and that has a good amount of stretch to it.
Get a piece of yarn or ribbon and tie it around the largest angle you can make around your foot, shown in green on the diagram below. Make sure you can slip your foot all the way through this loop.
Try your jeans on inside out and mark the center of the knee* with a pin.
Carefully remove and lay flat on the floor.
One seam on each leg will have top stitching, and the other will be left plain. The un-top-stitched seam is the seam you will be moving. It will probably be the outside seam.
Making sure the target seam is absolutely flat on the floor, mark a point on the old seam 6 inches above the knee mark you made earlier. Put another pin 2" above the hem. Lay the yarn loop you made earlier against the OPPOSITE seam, level with the 2" pin, and measure straight across. Add about 1/2" for safety, and put a vertical pin at that point.
With a pencil trace a faint line along where the new seam will be, from the vertical pin up to the one above the knee. Between the hem and the 2" pin, your line should be straight. Then it will go diagonally up towards the knee. Between the knee and the 6" the new seam should be very minimal, and trail off very very gradually into the old seam. The diagram above exaggerates the slope a bit (shown in red) for illustrative purposes. You want your line to be as smooth as possible, almost straight, but just with a little sway to it at the top to taper off.
Optional: use a seam ripper to take out the hem for a couple of inches on either side of the new seam line. This will allow you to resew the hem properly, rather than sewing through the already folded hem.
Baste along your pin line. Try the jeans on and check that everything looks okay. Redo if necessary.
When you are satisfied, fold the other leg over the first, carefully aligning the seams. Stick pins through the basting line on the finished leg through to the other leg. Carefully flip the jeans over and stick pins through the new leg at the point where the first batch of pins is sticking through. Carefully peel the other leg off, leaving the new pins behind to mark the stitching line. Mark/baste as before.
Use a sewing machine to carefully stitch just inside of your basting line, so that your basting stitches are buried in the new seam allowance. Cut off the excess fabric (leaving 1/4-1/2" of fabric) and zig zag along the edge to prevent fraying.
If you undid the hem, refold it and stitch closed. Optional: use matching embroidery floss to redo the top stitching.
* NOTE: If you use jeans that are not snug in the thighs, or that are snug down past the knee, you will need to move your "knee" pin above up or down accordingly. The "knee" pin should be at the point where the leg is starting to loosen, with maybe an inch or two of extra fabric.