Take measurements wearing only baselayers/light clothing. The tape should be tight but not restrictive.
1) With a tape measure, measure your chest at the fullest part, keeping the tape horizontal.
2) Measure your waist at the narrowest point above the pelvis keeping the tape horizontal.
3) (For women) With your feet shoulder width apart, measure your hips at the fullest point while keeping the tape horizontal.

Men's Clothing