I can never overstress the importance of a mockup for your costume!
Also called a toile, this is the first draft and sometimes pattern for your garment! It’s especially important if your costume has abnormal seam placement or you don’t fit neatly into commercial pattern sizing (and let’s face it, most people don’t!) Making a mockup can save you so many headaches later: you can check the fit of your pieces, make rounds of adjustments, and not worry about messing up your nice and sometimes expensive final materials.
A mockup should also use material that emulates the hand, drape, and thickness of your final material. For example: on my Black Widow bodysuit, I used a super cheap stretch knit, because my final fabrics were all stretch neoprenes. For my Astarion breastplate, the final was going to be veg tan leather, so I mocked up in craft foam (I have also used felt as a leather mockup material!)
Mockups can be lengthy processes; I often end up doing 2-3 rounds of fitting and alterations before I’m happy with the final pieces. It’s frustrating at times to work through the fitting challenges, but ultimately I have always regretted the times I didn’t do a mockup, and I know my final costume will be better for having taken the time and care!