I had a pair of jeans that had gotten a rip in the knee, as well as stretched out in the shins. A while ago I think I was watching some movie or television show (I don't remember which) and liked the look of the skirt a character was wearing, and I was thinking of getting rid of the jeans when I remembered that thought.
Take your worn out pair of jeans...
...and cut the legs off about an inch, inch and a half below where you want your skirt to end. I recommend trying them on and drawing on the jeans in pencil.
Put some music on and sit somewhere comfortable, because this takes some time...Take your (maybe terrifying-looking) seam-ripper, and cut the seams along the INNER thigh and crotch of the pants.
This is what it looked like when I finished.
I was trying to avoid the awkwardly pronounced seam I've seen in other jean-skirts, so I made a little cut at the top and trimmed off the hem of the inner thigh.
Fold the edge of the leg that you want to be in front, and then layer it over the edge of the other leg. Pin this down. Now, carefully turn the skirt inside out. This is kinda tough to explain, but the way I pinned the legs, there are three layers of denim. (Two for the folded top leg, and one for the bottom.) However, I only wanted to sew two of them (the bottom leg, and the part of the top leg that was folded under) so that my seam wouldn't be very visible. I sewed these two layers together, and separated them from the third as I went by pinching the two layers and pulling the pins out as I sewed.
This is what it looked like when I finished the front.
Do the same thing in the back, except you'll have to do less length because the original seam comes down farther.
Looks like this when done!
Fold the edges of the jeans inside the skirt, and sew to make a hem. At this point, trim any excess string from inside the skirt, or if the new seams are too bulky trim those a bit too. Then you're done, wear and enjoy!