You can use a mallet or flat side of a meat tenderizer to pound the chicken breasts to 1/4-inch thick. You'll want to do this in between sheets of parchment paper or plastic wrap to minimize any mess from the chicken juice splatter. Don't be too aggressive when pounding them as the chicken can tear. If your chicken breasts are thick, instead of pounding them, you can butterfly them or cut them in half horizontally.