When I was doing it first what I did was make my own pattern. I draw rhombus and then I place it on fabric. Then I cut it out. You could use 2 different kinds of patterns (like I did).
If you drav exactly size of rhombus, it will be smaller when you sew it together
Then these rhombus join together and sew it with each other. So you could have normal fabric/ material.
Then you cut out (pattern) fabric for t-shirt =) you could use som efrom
Easy, isn't it?