Turn a pre-washed shirt inside out. Lay a sheet of freezer paper (shiny side down) on the backside of the surface you plan to paint. Iron the paper onto the t-shirt using an iron that's set to medium (no steam). This is going to protect the backside of the fabric. Then carefully turn the t-shirt right side out.
You'll want to draw and cut out a stencil using freezer paper. Lay the stencil shiny side down, and carefully iron the stencil onto the t-shirt. If you are ironing on letters or small individual pieces, do them one by one.
Check to make sure the edges are well-adhered to the t-shirt, as this is what makes a good clean line once you remove the stencil.
You'll want to remove the stencil while the paint is still wet. Carefully pull on the stencil to remove it, being careful not to get any paint from your fingers onto the t-shirt. (Note: I've done this when the paint has dried as well, but I find that the lines tend to be cleaner when the paint has not dried, especially if you've used several coats of paint.)