Use the tape to measure around the base of your neck, where it meets your shoulders. Put a finger between your neck and the tape measure for a more loose fit collar.
This measurement should be taken beneath your armpits, around the widest part of your chest/bust and shoulder blades. Make sure to keep the tape measure horizontal, and don't pull to tight or hold your breath while measuring.
MEN: Put the tape around your natural waistline, which should be to your bellybutton. Put one finger between the tape and your body before you measure.
WOMEN: Measure around your natural waistline. This is the narrow part of your waist, about an inch above your navel. Relax and breath out before you measure.
This is measured between your groin and your lower ankle. You may find that it's easier measured on a pair of trousers that already fit you.