Lay on a flat bench, dumbbells in hand. Form a straight line with your arms through your shoulders, maintaining a slight bent in your arm, palms facing up. Bring the weights up by squeezing your chest, exhale, bring the weight down, inhale.
As with other exercises, let’s first focus on what will hurt you. Going too far down is the most common way. Your upper arms should never go more than 10 degrees lower from making a straight line through your shoulders. Doing this exercise too quickly is another way to get hurt. Unless you want to tear your pecs, control the entire motion of the exercise. Do not use heavy weights. This doesn't mean that you shouldn't go heavier as you build your chest, but don't go heavy and sacrifice form. Maintain only a slight bent at your elbows. The greater the bent, the more this exercise is like a dumbbell press and not less like a dumbbell fly.
You could lay on the bench and keep your feet flat on the floor or the bench. Keeping your feet on the bench will alleviate some of the pressure on your lower back and is recommended for people with lower back problems. In either position, you should support your lower back by squeezing your abs. Bring the weight up over your chest. Remember, where you bring the weight is where the exercise will focus. We find the best squeeze when we lift the weight over the lower chest. At the top, you may extend your arms out, but don't lock out your elbows. Reprise the slight bend as soon as you start lower the weights again. Even though this exercise makes a greater use of your shoulders and appears to require them to move, the only parts that should be moving are your arms. Keep your shoulders flat on the bench throughout the exercise