I've used conventional patterns like this (Simplicity 4401):
Modify as needed - lower top edge to be on hips, make it a size smaller to be fitted (or try on inside-out and mark seams that you want to take in), leave a seam open to above knee in front for a slit, etc.
On this pattern, I wouldn't want seams in the center front and center back, so would modify those pieces.
Sounds more difficult than it is
