There are burger press tools on the market specifically made for smashing burgers, and these are ideal because they allow you to apply pressure evenly over the surface of the patty. They are available online or at bbq or kitchen retailers. You can also use a large metal spatula, and press down firmly in the center of the spatula as you smash the burgers. Don't use spatulas that have holes or openings in the middle though as the burger will push up through the openings and stick to the spatula.