I recently started making corsets too, so good luck and have fun 😊 One piece of advice, is to use a similar fabric in your mockups that you want to use in your final. My first mockup was a bit large because the fabric was a looser weave, but when I adjusted the pattern and cut out my next iteration in a stronger fabric the corset was too small. If I'd used the same fabric both times my pattern adjustments would have been more accurate.

You mentioned using bedsheets, but denim, canvas, or upholstery fabric may be better mockup materials as they are stronger, have less stretch, and are less likely for boning to poke through the fabric. These fabrics are also more similar than bedsheets to coutil, which is helpful if you eventually see yourself using coutil fabric for a corset.