I recently draft one for a jean. Just to show off I also successfully manage to sew the muslin (as successful as a newbie sewer can be)
One thing for sure I really have to study how to sew the fly and zipper before I draft the pieces. The pattern for the flies itself is simple but there is a lot of variation, like whether you want the fly facing to be one-piece with the trouser pattern or separate piece, the shape of fly shield/tab, seam allowances etc.
Or you can do the other way around. Make the pattern first following textbook instruction then figure out how to sew it.