Measure around waist, where you normally wear your pants. Measure over shirt, but not over slacks or belt.
Measure at the fullest part of your hips. Make sure tape is comfortable and not too tight.
Measure from the crotch along inseam leg to desired length. (Wear the shoes you intend to wear the pants with.) If you have a similar pair of well-fitting pants, lay them flat, measure along inseam from crotch to bottom of hem.