Measuring for an overbust corset is similar to measuring for a bra size. Here are the steps to measure for an overbust corset:

1. Wear a well-fitting bra and remove any bulky clothing that may affect your measurements.
2. Measure your underbust: Wrap a measuring tape around your ribcage, just under your bust. Make sure the tape is snug but not too tight. This measurement will determine the band size of your corset.
3. Measure your bust: Wrap the measuring tape around the fullest part of your bust, usually at nipple level. Make sure the tape is snug but not too tight. This measurement will determine the cup size of your corset.
4. Subtract your underbust measurement from your bust measurement to determine your cup size. Use this measurement to find your appropriate corset size.
5. When trying on the corset, make sure it fits snugly around your bust and underbust, with no gaps or bulges.

Keep in mind that corset sizing can vary between brands, so it’s important to refer to the specific brand’s size chart for the most accurate fit.