Loungewear never looked so good. A drawstring waistband lets you wear these lightweight jersey pants as fitted, or as loose, as you want.
Measure at the natural waist, and keep tape comfortably loose.
Stand with heels together, and measure around the fullest part of the hips.
Measure a similar style pant that fits well and is the desired length. Measure from the crotch seam to the hem.