Each brands size guide can be found on the product page under size guide.

Bust - Measure the bust at the fullest part. Measure all around the body (total circumference).

Waist - Measure the waist where the body bends. It helps to bend side to side to identify exactly where to measure.  You can put an elastic band around the waist to mark the correct placement.

Hip - Measure the hips at the fullest part, usually around the seat.