Tricks keep pockets flat on pants, skirts. Tricks ... Cut a piece of stay tape or narrow selvage the length of the seam that attaches the pocket facing to the pant front. Use your paper pattern, ...

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 ...