Lay on a flat bench dumbbells, in hand. Bring your elbows out forming a straight line, lower arm perpendicular to the floor. Bring the weight up, exhale, bring it down, inhale.
First let’s get the things that hurt you most out of the way. As with other exercises, do not lock out your elbows. This means bring the weights up, and straighten your arm up to and until the point where your elbows locks. Do not bring the weight down too far. That hurts your shoulders. Going down until your upper arms make a straight line with your back is sufficient. You can bring the weights slightly lower to get a stretch and extend the range of motion.
When you lay down on the bench your feet should either be flat on the floor, or flat on the bench. Bringing feet flat on the bench helps alleviate the pressure on your lower back and is recommended for people with lower back problems. You upper back should be flat on the bench at all times. Focus on doing the exercise by moving your arms and contracting your chest - do not bring your shoulders forward. Always make a conscious effort to keep your shoulders pinned back. It is also important where you bring the weights up. Where you bring them up will determine the concentration of the exercise. That said, bring them up over your chest, NOT your shoulders. I feel the best squeeze by bringing the weights up directly over the lower chest.
There are three starting and finishing positions. You could have your palms facing forward, each other, or back. You could start with any one and finish with any other. The most common such combination is starting with palms facing forward and turning them towards each other as you press the weights up. Generally though, people just keep the palms facing forward both at the top and the bottom. When palms are facing up more emphasis is placed on the sternal head, i.e. the middle of the chest. That said, everyone's body is different so do the variation that feels best and yields most results for you.
Once you are lifting heavier weights it is easiest to begin the exercise by placing both dumbbells on your quads when you sit down on the bench. Then push them up with your legs as you lay flat on the bench.