Take the measurement around the fullest part of your bust. Make sure the tape is comfortably around the bust. Measure loosely, but make sure the tape does not slip down your back.
Wrap the measuring tape around your hip, snug, but not tight. This is your hip measurement.
The fullest part of your hips is usually about 8" below your natural waist (this may fluctuate from 6" to 9" depending on your height).
Measuring your bust will help determine the best loungewear top size for you.