It is always easier to have someone else measure for you; when measuring ourselves we tend to get inaccurate results, and this will affect the fit of your corset/garment. When in doubt, have a couple of people measure you, and see what ends up being the most accurate.
To start, you will need to be wearing non-bulky, comfortable clothing (I suggest a thin tee shirt and knit pants, or against skin when available; no constricting garments!) including your best-fitting, most comfortable bra. You will also need a flexible measuring tape and a length of elastic, ribbon, twine, lace, or thick string. Tie the string at your natural waist, where you are the smallest. Do not tie it tight enough to make a mark on your skin, rather simply resting against your skin without sagging. If you are unsure of where your natural waist lies, tie it around your midsection and bend your torso from side to side; this will make the elastic or ribbon settle into the correct area. This is not your pants waist! Most modern slacks are designed to rest at the top of your hips, measuring from there will distort your corset pattern.