Set up in front of a cable machine that is set at chest height.
Start by sitting on top of a stability ball facing the cable machine. Hold the handles of the cable machine with each hand.
Widen your base of support by by spreading your feet apart past your hips and then try to perform a chest press with both hands.
Push the weight away from your chest with both hands and stabilize your core and lower body at the same time.
The stability ball is going to force you to stabilize your lower body as your perform this exercise.