Wrap the tape measure around your chest and measure your entire bust. Measure the bust line at the fullest part of your chest. Pull the tape as tight as you can without changing the shape of your breasts. While you are being measured, your arms should be at your side and relaxed
Start at the top of your hip bone, then bring the tape measure all the way around, level with your belly button. Make sure it's not too tight and that it's straight. Don't hold your breath while measuring.
Measure from the top if your shoulder to where you want the dress to land. This can vary among women depending on their preference.