Start either in your skivvies, a tight fitting dress, or a tight tank top and leggings. Wear an unpadded bra, or no bra.
To measure your true bust, find the fullest part of your chest, and wrap the measuring tape around so that it is parallel to the floor - it should not go up higher or lower in the back, than it is in the front. To determine your band, measure below the bust, right around where a bra band would sit.
Measuring your waist is a little more tricky. A lot of people think that their waist is either a) where their pants sit, or b) where their belly button is. While that is sometimes the case, it is not always 100% accurate. To find the narrowest part of the waist, wrap a measuring tape around your waist - area, and bend from side to side. Wiggle Wiggle! The tape will end up nestled in the narrowest part of your waist. This measurement is your waist measurement.
The deadly sins of measuring include pulling the tape TOO TIGHT! I have to admit, it's tempting to pull round down to the lower inch, or suck in a little bit, but accurate measurements are so important. Plus, it's not the number that counts, it's how you carry it! Make sure you check yourself from multiple angles to avoid measuring too high or too low. Always keep your measuring tape paralell to the ground as much as possible - the tape will sometimes want to dip up or down in the back, which can add on inches.