Graphic from House Of Brides.
If you are being fitted for a gown you might want to have their seamstress do it but if you are sewing for yourself or just curious it’s pretty simple:
Get a measuring tape and do this in your underwear or lightweight clothing.
Wrap the tape around the fullest part of your bust. Don’t make it too tight and make sure the tape is flat, not twisted. You might want to have a pencil and paper nearby to jot this down.
Do this again for the smallest part of your waist, and the fullest part of your hips and thighs.
That is the simplest way and if you need even more detail you can check out this article:
Find your size at Fashion on About.com.