To what Michelle has written I'd like to add my thoughts:
- Add a 1" additional side seam allowance
- Add a 1" additonal waist seam allowance
- If you have more than an "average" (whatever that is) size butt, your back crotch curve should look more like a "J" or possibly even an "L"
- After sewing zipper, darts, back and inseams, pin-fit side seams to your liking
- Any excess seam allowance your remove from the waist or side seams, remove that same amount from your pattern pieces
- Have a good pants fitting reference book handy. I prefer "Sewing Pants That Fit" from the Singer Sewing Reference Library.
- Be patient, it may take making more than one muslin to get what you want.
I wanted to add just a simple 1" waistband to my slacks but wanted one that wouldn't bend. I used 1" not-roll elastic and absolutely love it! I'll do a simple tutorial in the next day or so. You are going to love it!