You are buying a pair of Jeanswest Boyfriend Jeans. Size 12. Compare to similar jeans or double waist and thigh measurements to compare to your body. If style is Low rise or Mid rise measure around the middle of your hip bone.
The waist is measured by laying the jeans flat and measuring straight across the top. That number is then doubled. The inseam is from the bottom of the jeans, running up the inside of the seam of the leg until it meets the crotch.