This is an isolation exercise, which means the muscle involved is only the chest. In a chest press, the arms bend and straighten, so engaging the triceps, the muscles down the back of the arm, too. Because a chest fly is an isolation exercise, the weight will be much lighter than in a chest press.
It is best done with the feet up. This better engages the chest.
Keep the arms long. There can be a slight bend in the elbows, but this bend should not change during the movement. There is a tendency to bend the elbows more at the lowest point, as if pulling the dumbbells into you. And so it helps to keep reaching wide through the arms from the middle of the chest.
How low the arms go is a moot point. If the shoulders are stable, then lower is better. If there is any pain or fear, then only go as low as there is no pain – and use this as a warning to sort the problem out with high quality massage, good stretching and attention to form.
The back stays flat throughout the movement. Use the low abs to stabilise the body – that is what they are for.
This exercise should be felt in the chest. It is often a matter of finding a thought that helps this: hug a tree; hug a huge Swiss ball; squeeze the ceiling; squeeze the world. Use whatever works on the day.