On all fours, hands slightly wider than shoulders and feet close together but not touching, straight line from ankles to head, squeeze glutes and contract your abdominal muscles as if you were expecting to be punched in the stomach.
In a controlled manor focusing your mind on both your chest and triceps, lower yourself till your chest almost meets the floor and then push yourself back to the start position. To make the exercise easier, you can perform an inclined bench press with your arms on a bench or even your bed. To make it more difficult, you can do a "Decline Pushup
where you put your feet on a bench or a table and hands on the floor. The higher your feet
the more difficult it becomes and the more you engage your shoulders and back too."