Use a measuring tape, over your undergarments.
You can also use a string, and then line it up to a yardstick.

BUST Measure under your arms, around the fullest part of your bust.

WAIST Measure around your waist at narrowest point (usually about your navel).
Keep one finger between the tape and your body.

HIPS Stand with your heels together and measure around the fullest part of your hips.