I'd like to add to Kenneth and JDtailor's techniques. The inseam of the back pants leg needs to be shorter than the inseam of the front pants leg. Before stitching the back leg inseam to the front leg inseam, stretch and steam press the back leg inseam approximately 7" to 9" from the crotch seam to above the knee area. The stretching and pressing builds in shaping below the buttocks. You can consult a tailoring book for photos on this technique. Stitch the inseams and proceed with sewing the pants
In response to mercystreet and the problem with "creepy crotch", it sounds like the crotch curve needs to be adjusted. Many times this occurs because the crotch curve is too straight or not deep enough. I've resolved "creepy crotch" by putting my pants on and carefully pinning out the excess fabric at the crotch curve. Its easier to have another person pin out the excess