Use a tape measure. For more accurate measurements, measure directly on the body. Keep a pen and paper ready and write down the measurements.
Bust
Measure 1:
Measure around your ribcage, directly under your bust and level all around.
Measure 2:
Measure over the fullest part of your bust, keeping the tape level around your body.
Waist
Measure around your waistline, hold a finger between your waist and the tape to get an exact fit.
Hips
Stand with your feet together and measure around the fullest part of your hips, about 20 cm under your waist.