The chest dip is a variation of the tricep dip that focuses more on your chest. Even though it uses a dip machine, it is a body weight only exercise.
Steps
Grip the bars of the dip bar (ask for assistance if you're not sure which machine this is) and push yourself up to the starting position. Your arms should be straight and fairly close to your body. Keep your knees bent so that your feet never touch the ground.
Slowly lower your body, leading with your chest as you go down. Your chest should start pointing down towards the ground at a 45 degree angle. Lower your body until you feel a stretch in your chest and shoulders, then begin to slowly raise your body up again.
Tips
If you can't quite manage this exercise then you can get a spotter to hold on to your feet and assist you. If the exercise is too easy, then you can wear a weighted belt.
Try not to lock your elbows in the starting position.
Keep your abs tight and back straight throughout this exercise.