Usually when I made a skirt from jeans I just cut straight across right at the crotch and add some fabric below that. But the other day at my Girl Scout meeting one of my Girl Scouts was wearing a jean skirt she made from a pair of jeans. Hers were made completely out of the jeans and no extra fabric but I wanted to use pink fabric for Hannah because she's a huge fan of pink.
Then I cut the inside seam and the bottom hem of the pant leg I had cut off, leaving me with a rectangle of fabric:
Then cut a a piece of fabric 1 1/2 times larger than the bottom of your pant skirt and as long as you want your skirt to be (maybe try on the jean skirt and figure out how much longer you want it to be, Hannah's skirt goes to her mid-shin):
Hem the bottom of your skirt when you're finished. And if you want, add some appliques: