Take the t-shirt and stretch it over the foam board.
Take your stencil; apply a light coat of spray adhesive to the back of the stencil. If you put too much adhesive on, it will mess up your shirt.
Lay down your stencil on the t-shirt wherever you want it to be.
Take the acrylic paint and apply it to the foam roller. Roll the paint in a light coat over the stencil.
Use the hair dryer to dry the coat of paint. Make sure that the paint is all the way dry. Repeat Step 4 as needed (light colors on darker shirts will take 3 to 4 coats).
Once the paint is on the shirt to your satisfaction, take the stencil off (make sure the paint is completely dry first).
Wear the shirt.