Before you start measuring... remember, size is just a number. Don't suck in or pull the tape measure tight. Stand in a relaxed position, let it all hang out and measure with a taught, but not tight tape measure. It's better to get the RIGHT size than worry about what the size is.
WAIST: Measure the narrowest point on your torso, around your waist.
HIPS: Measure around the widest part of your hips.
INSEAM: Measure on the highest point of the inside leg to the ankle bone.
Use these measurements with the chart below to get your estimated size.