I had some old Tripp NYC jeans (like, YEARS old) that I certainly could not wear anymore. But I loved the patterns, so I made skirts. :) I only took a picture of the pink leopard print one but I also have one made out of traffic cone orange pants. And I applied similar methods to turn a skort into a skirt. I originally did this by hand so it took ages, but I did the second pair with a sewing machine and lemme tell you! It was MUCH easier. (Also: ignore the weird floor in the picture haha. I was in the bathroom because it has the best light in the house-go figure.)
Turn le jeans inside out!
Cut along the inseam. Or use a seam ripper if you have one, and simply tear the inseam apart (it makes life easier in the long run.)
Cut/ rip all the way up. Do it on both sides.
Cut the weird little seat connecting seam. Or rip it open with the seam ripper.
Pin the skirt together in the front and in the back. Straight pins are ideal (it's easier to remove them while sewing), safety pins will work.
In the back you'll have this weird flap. Cut it and sew the skirt, or fold it in and sew, then cut. Whatever you do, make sure you check the skirt for weird folds before sewing. If you have them you probably need to cut/rip the original seam a little more. Get rid of the curved section. I suggest using a couple of line stitches to secure the fabric. Zigzag Stitches (sewing machines!) make it extra secure. Even the hem as necessary by cutting or sewing and you're done!