Step 1:

The Waist

Lay the jeans flat & measure straight across the waist.

Then take that number and multiply it by 2.

(For example: 13 x 2 = 26")

Step 2:

The Hips

Measure straight across the hips. Right beneath the zipper.

Step 3:

The Rise

Measure from the top of the crotch to the top of the jeans.

This tells you how high they will fit on you.