The exercise is very similar to the standard bench press (Guide found Here), with a few variations.
1. Warm up and stretch thoroughly. It is important to stretch the shoulders completely to avoid injury. Try arm circles, and then arm circles holding added weight (5-10lbs).
2. Begin by lying down on the flat bench under the barbell with your feet planted firmly on the ground and shoulders touching the bench. Grip the bar at about shoulder width, with about 12 inches between your hands. (You can adjust the grip to make it narrower, but this tends to cause the elbows and shoulders to flare out and negates from the lift- Don't bring your hands too close together (works shoulders) or too far apart (works chest). Keep the elbows bent at 90 degrees and the hands locked tightly around the bar with the thumbs on the outside.
3. Lift the bar off the rack and slowly lower it just below your chest - about the lower sternum area. Aim for a count of two seconds during this process. Make sure to inhale as you lower and stop once the bar reaches your chest. Do NOT drop it rapidly or bounce it off your chest.
4. After a brief pause, exhale sharply and force the bar back to it's starting position. Try to take at least one second to do so. Focus on squeezing the muscles of the arms and not just pushing with the chest. It should be felt in your triceps. Keep the feet firmly planted and hold your waist steady; do not squirm on the bench. To help, visualize the bar staying in place, and you pushing your body away from the bar instead. It will assist with movement.
5. Repeat this process for the defined number of repetitions in one set. Repeat for up to five sets. Ideally you should begin light to warm up, and slowly add weight. Set 4 should be the heaviest, then drop weight as strength fails and try one more light set.