To measure, hold measuring tape level to the floor for bust, waist, hip measurements.
Bust: measure over the fullest part of the bustline, keeping the measuring tape level across the back.
Waist: measure around the narrowest part of the trunk, a few inches above the belly button. This is higher than where pants are generally worn.
Hips: measure around the largest part of the bottom, keeping the measuring tape level across the front of the body.
Torso: hold one end of the measuring tape level on the flat top of the shoulder (where a strap would cross). Pull the tape diagonally down through the crotch and up again diagonally across the back to meet the measuring tape at the shoulder again. This is the torso measurement.
Keep in mind that swim lycra is stretchy and an approximate measurement will fit very well. Select the measurement that most nearly matches your body.