For starters you need to use a pattern with a well-fitting crotch. My three favorites are Vogue 7027 for a tailored trouser, Vogue 7179 for a pleatless pant with a faced waistband, and Butterick ...

This pair of approximately 3,000-year-old pants, the oldest ever found, displays weaving techniques and decorative patterns that were influenced by cultures across Asia, researchers say.