You can start with a regular pattern such as the skirt (left photo) for Simplicity 4401:
If possible, buy some inexpensive stretch fabric to test it out - sew up the all of the vertical seams, turn it inside out, and try it on. (If you're using stretch fabric, you won't need the zipper.) Pin whatever needs to be taken in, fold down the top to where you want it to hit your hips, etc. Take it off and re-sew. Try it on again, etc. I've gone through this process with an existing pattern to get what I want, then have cut out my new pattern from interfacing. You'll need to make a casing at the top for elastic. Leave a seam open (or not) from just above the knee down for a slit. Have a friend help you level it out at the bottom and hem it (or not if the fabric doesn't fray!)
Good luck!