Put the measurements tape around these areas:
Bust - Measure around the widest part of your chest
Waist - Measure around the narrowest point of your waist
Hips - Measure around the widest part of you hip area.
Hem - For dresses, measure from the collar bone at the back of your neck down to where you want the hem to finish, or down to floor for long dresses or skirts. Don't forget to wear the kind of shoes you will wear with the item.