Use a basic tape measure (like the ones pictured) for the easiest, most straight-forward way.

If you don’t have a tape measure on hand, you can use a ribbon, soft cord, string, etc. You don’t want to use something elastic/stretchy, or too stiff like an electrical cord as those can give you inaccurate results. Measure yourself with the ribbon/string, and then measure that with a ruler or other measuring device. There you go!

Be sure to not pull too tight around your body, and I’d recommend not measuring over clothes. Measuring against the skin or over-top undergarments (e.g., over a bra you’d normally wear to get a bust measurement) would be best imo.

I use inches for measurements across my shop, however when submitting yours, you can do so in either inches or centimeters.