Right now I'm working away on drafting a jeans muslin/toile . I'm very happy I chose to sew my jeans-style skirt first. It helped me to divide up the challenges of a new-to-me sewing project.
It definitely gave me a feel for the heavy denim I want to use, topstitching, and hardware. Plus.... A week after completing this poroject, I've worn the skirt five days, and gotten a kick out of it every time. I really feel ready to tackle those pants inspired by Mr. Levi.
You can certainly fit a jeans pattern, by following instructions in a commercial jeans pattern. Several of the more popular patterns have special instructions just for doing this, and I made a collection of several of those patterns over the last year. However I'm lucky enough to be able to take Lynda Maynard's "Pants Drafting" class at Cañada College, so I'm taking advantage of that opportunity to get my jeans pattern created that way, and learn something about pattern drafting along the way.
We're using the book Building Patterns, The Architecture of Women's Clothing by Suzy Furrer. I'm kind of excited to look at the variety of garment blocks shown in the book!