To measure your waist:
1: First find your natural waist. This is the point between your lowest ribs and your hip bones. Easy to identify by placing your hands on your hips.
2: Place the tape around the waist line.
3: Relax your tummy by breathing out gently and do not tuck in. adjust the tape so that it is level around your waist line and measure.
4: Remove the tape and record your result.
Alas sizes for women just aren't that simple. Different cuts and different manufacturing methods will mean that a Size 14 in one shop may be significantly different from a Size 14 from another. I have a friend who managed to buy a Size 8 pair of trousers in one shop, but came up a Size 14 in another, which goes to show the silly levels of variation between suppliers.