Measurements should be taken directly on your body.

Neck: Measure around the base of the neck where the collar sits.
Chest: Measure under your arms, around the fullest part of your chest.
Waist: Measure the narrowest part of your waist.
Seat: To find your seat, measure 20cm from your natural waist as a guide. Stand with your feet together and measure around your widest part.
Inside leg: Measure from the top of your inner tight to your ankle bone.