Pat the chicken dry with paper towels. Place on a large cutting board, breast side down with the neck facing toward you.
Using good-quality kitchen shears, hold the neck and cut along one side of the spine, separating it from the ribs. Be sure to cut as close to the spine as possible. Repeat on the other side of the spine. If you are having difficulty, rotate the bird so the tail faces you and cut from the other side.
Flip the chicken breast-side up. Using the palms of your hands, press along the breastbone; you might hear a crack. This should flatten the chicken completely.