I read that it is helpful to make a toile (also referred to as a muslin) before starting your actual garment. You essentially create a mockup in cheap fabric before you commit to the good stuff. It’s supposed to reduce waste of your precious fabric and, if you do it right, give you a better fit.
Some people rely on finished measurements supplied with a pattern and skip the toile altogether, but I decided to take the extra step. I really struggle with standard bust measurements in shops, so I liked the idea of being able to see how it fit before I made the commitment.