Tutorial: T-Shirt Texture with Opacity Masks in Adobe Illustrator
Textures shown are the Plastisol 2 textures from T-Shirt Design Master Collection 1.
Using the Opacity Mask method will knock transparent texture through your designs in Illustrator.
These textures will show up when you output color separations for screen printing.
Setting up your art with this method also works great for DTG printing and sublimation.