Place the chicken breast on a cutting board, smooth side down (where the skin would have been). Place one hand on top of the chicken, making sure to curve your fingertips up slightly so you don't accidentally cut them. Insert the knife into the middle of the thickest part (the top round bit) and slice the chicken breast horizontally into two even pieces. Then, sandwich the sliced chicken breast between sheets of plastic or parchment paper and pound them to an even form and thickness, about 1/4 inch thick.