Set a bench to a slight decline, then grab a pair of dumbbells and lie down. Position the dumbbells over your shoulders, with your arms fully extended. Your arms should be positioned so they are perpendicular to the floor and your palms should be facing each other.
Keeping your elbows still, lower the dumbbells by bending your arms until your forearms are slightly below parallel to the floor. The dumbbells should finish beside your head.
Squeeze your triceps to lift the weight to the starting position with your arms fully extended. Keep your elbows tight and don't let them move as you extend your arms.
Pause at the top and repeat.