Second time making a body suit but I had difficulty with my first one. I've used Yaya Han bodysuit pattern and her 4-way vinyl fabrics but this time I don't want to use the final product too early. What do you recommend to use as a mock up? Spandex or muslin fabric?
Use whichever of the two has a similar 4-way stretch to yayas. You want to get as close to your final fabric as you can, so you encounter the same effect or issues that may come up.