With siccors. You know what one looks like just cut a t shirt to look like one. Or whatever you want to make look like one.
If you already have or purchased a muscle shirt, try to make a pattern from it. Designing and creating a pattern is a very special skill due to the form of each individual. You can lay a shirt on some plain paper and draw each section as best you can. You can spruce up lines after you remove the shirt. Remember to add seam allowances to your drawings.