Use measuring tape (use inches) and measure where you would like your beads to fall along your waist, or hips.
Make sure you are relaxed, NOT sucking in your stomach. You want to get the most accurate measurements so that your beads are not too tight. You should keep in mind that they become tighter due to monthly bloating, weight gain, and/or becoming full/over eating.
If you are using your beads to help track your weight, measure above the belly button.