The Iron Gym is a so-called "door gym," a leverage-mount pullup bar that doubles as a base for several other bodyweight exercises. You don't actually need the Iron Gym in order to do pushups correctly; in fact, pushups require no specific equipment at all. But using the Iron Gym as a pushup bar can help reduce the wrist discomfort that sometimes develops from doing pushups on the floor.
Place the Iron Gym on the floor, with the shorter of the two horizontal bars facing toward you and the longer bar -- the bar you'd grasp to do pullups -- facing away from you.
Grasp the two parallel arches on top of the Iron Gym at the high points. Your palms should face each other.
Adjust your body positioning so that your chest is directly over the point where your hands make contact with the Iron Gym. Straighten your legs so that your body weight is balanced on your hands and the balls of your feet. Your body should be straight like a board.
Bend your arms, lowering your chest toward the Iron Gym. For a conservative range of motion, stop when your shoulders are level with your elbows. If you regularly do sports or other movements that require you to push something away from close to your body, you may find a slightly deeper range of motion to be beneficial.
Extend your arms, pressing your body away from the Iron Gym. Squeeze your abs to keep your body flat throughout the motion.