Waist
Measure around your waist where you typically wear your trousers. Keep the tape measure tight against the body as this should be snug but not uncomfortable.
Hip
Measure around the fullest part of your hips, which is generally over the lower part of your hip bone and over the fullest part of your seat. Keep the tape measure snug, but not tight and parallel to the ground.
Thigh
Measure around the fullest part of your thigh. Keep the tape measure snug, but not tight.