1 - Bust
Measure under your armpits, around your shoulder blades, and over the fullest part of your bust.
Don't pull the tape measure too hard. For fitting a bra, you also need the measurement just under your bust.

2 - Waist
Measure around your natural waistline. This is the narrow part of your waist, approximately an inch above your navel.
Relax and breath out before you measure.

3 - Hip
The hip should be measured around its fullest part (about 8 inches below your waist).

4 - Inseam
This is the measurement groin to your ankle. This can also be measured on a pair of trousers with the proper length.