Start with a pair of jeans that currently fits you in the waist. It does not have to be wide leg; anything will do, including capris. Personally, I prefer to get cheap jeans at a thrift store or garage sale. If you want a long skirt, you will need another pair of jeans in a similar color (you are going to cut them, so they don't have to fit) or a piece of fabric that you like. You'll also need matching thread, I usually use brown or cream. Here are some samples:
1. If making a short skirt, cut the legs off at desired length (remember to add an inch or two for hemming). If making a long skirt, skip this step.
6. Sew the pieces together following the original stitch line of the pants. do two rows of stitching to prevent fraying.
After you wash it, you will need to flip it inside out to trim all the frayed threads. You may need to do this a couple times.