The "waist" location is different for everyone.
To find the waist:
On a slim form, have the person bends side to side, the waist will create crease lines, this is usually a couple of inches above the belly button.
On a round form, the waist should be measured around the widest part of the stomach, usually on, or below, the belly button.
Pro-tip: people tend to tense their stomach at this measurement, get them to relax their stomach.