Measure around the largest area of your bust which is generally over the bust point. Ensure that the tape measure is level (you can check this by standing sideways to a mirror).
Measure around the narrowest part of your waist, ensure that the tape measure stays level by checking in a mirror.
Stand with feet hip distance apart and measure around the fullest part of the hip and bottom area. Again, checking that the tape measure stays level and parallel to the floor to ensure correct the measurement is taken.