You make need to ask someone to help or stand in front of a mirror. Wear something similar to what you will be wearing under the garment and heels if you are going to do so on the day. You can measure in centimetres or inches.

Bust is measured at the largest part around the nipples

Waist is measure around the belly button (the smallest part of your middle)

Hips are measured around the lowest part including your bottom

Strap front is measured from top of shoulder to the top of your armpit (usually where your bra strap meets the cup)

Strap back is from the top of the shoulder to the top of the bra

Height is from back of neck to waist and from wait to where you wish the dress to be hemmed (remember your heels!)