Sit on a bench with back support and hold two dumbbells in front of you with hands to your side at upper chest level. Exhale and begin pushing both arms up until fully extended. While keeping one arm extended, inhale as you begin to lower the opposite arm to the original position. Repeat this movement for all reps all on one arm, then keep that arm above your head and repeat the movement for the opposite arm until all reps are complete.
Helpful tips and Tricks
Keep your head, back, & chest in proper alignment. The alternate arm that is not lifting should remain above your head.