Step 3: Attach the stencil to the shirt
Optional but recommended step
Take a piece of blank freezer paper at least as large as your stencil design and put it inside the shirt with the waxy side up, about where the design will go. Smooth out any wrinkles. Once the iron is hot, slowly run the iron over the front of the shirt. The waxy surface of the paper inside the shirt will melt slightly and make it adhere to the shirt. This takes more heat and time than you'd expect, so be patient. Lift up the edge of the shirt and see if the freezer paper peels away easily if you pull on it. If it does, use the iron for a few more minutes.
Alternately you can just slip a large piece of cardboard or heavy paper inside the shirt to prevent paint bleeding through to the back, but this piece of freezer paper provides a stable surface for painting and makes it easier to iron the stencil to the front of the shirt.
Iron on the stencil
Once the backer paper is in place, put your stencil on top of the shirt waxy side down. Make sure you have it positioned exactly where you want it, then begin to iron it. Pay extra attention to the edges of the design. You want the freezer paper adhered as smoothly as possible so that no paint will get under the edges.