If you are doing this exercise for the first time I DO NOT recommend you use a medicine ball. Try to get a feel for it and your limits using a lighter rubber ball or even a throw pillow in your living room. Please, please don't smash your face ;)
Start off laying with your back on the floor, knees bent and feet flat on the floor and a medicine ball in your hands rested above your chest.
Forcefully extend your elbows up and push the ball up directly above your body as high as possible
Follow through feeling a stretch on your triceps then catch the ball as it comes back down
Repeat for as many reps and sets as desired
To add a core element to this, you can also toss a stability ball, it will be more of a challenge to stay flat on your back.