Flyes are the most stimulating chest movement, as the arms follow the exact path dictated by a the contraction of the pectorals. Hold your arms by your sides and relax everything, then contract only the pecs. You will see that the arms travel in front of you, towards each other. This is the natural movement created by the chest. Flyes are designed to replicated this motion, but with added resistance. It is done lying down to be able to move the resistance against a force, gravity. So lying down, with a dumbbell in either hand,  arms about perpendicular to the torso, contract the pectorals. The exact angle of the arms to the torso differs amongst everyone, but tends to be a little below 90 degrees, eliminating shoulder involvement (you figure this out by seeing which position allows you to contract only the pectorals). Stretch the chest as wide as it goes, then squeeze it to bring the DBs back to the starting position.