Keep the rep timing slow and control the weight during the set. It should take about twice as long to lower the dumbbells than raise them.
Dont touch the dumbbells together at the top of each rep.
Use a full range of motion by allowing the dumbbells to drop down as far as possible on each rep.
Its best NOT to drop the dumbbells when you are done with your set. This can be hazardous to your shoulders and others nearby.
Instructions
Set a decline bench to and angle of around 30 degrees.
Grasp a pair of dumbbells and position yourself at the high end of the bench with your legs in the padding.
Slowly lay back on the decline bench and push the dumbbells straight up above your lower chest. Alternatively you could have someone pass you the dumbbells. This is the starting position for the exercise.
Slowly lower the dumbbells in a triangle shape until the handles are about level with your chest.
Pause, then slowly raise the weight back up without locking your elbows out at the top.