For this exercise, you need two dumbbells and a flat bench. Use a light weight until you become familiar with the exercise.
With a dumbbell in each hand, lie on your back on the bench. Put your feet flat on the bench, with your knees bent. Hold the dumbbells above your chest, arms straight but not locked, with your palms facing forward. Tighten your abdominal muscles so your back is flat against the bench.
Slowly, and with control, lower the dumbbells to your chest until your elbows are at shoulder level. Press the dumbbells up until the arms are straight but not locked. As you press up, hold your shoulder blades together. Count one-two-three-four as you lower the dumbbells, then one-two as you raise them. One-two-three-four. One-two.
When you are comfortable with the exercise, gradually increase the weight of the dumbbells until you can do 8 to 10 repetitions using proper form. Once you can achieve 12 or more repetitions, increase the weight to bring you back to the 8-to-10-repetition target zone.