1) Lay horizontally on a bench with your feet at shoulder width apart and planted on the floor. Keep your knees at a 90° angle. Your head needs to be off the bench. Pull your shoulder blades back.
2) Hold the dumbbell with both hands above your chest with your arms almost fully extended with a slight bend in your elbows. Have your hands on top of each other and your palms facing upward.
3) Inhale and move your the dumbbell backwards past your head while keeping your arms extended. Reach back as far as you comfortably can whilst making sure that you feel a stretch in your lats and chest.
4) Exhale and bring the weight upwards whilst keeping your arms extended. Bring the dumbbell back to your starting position with the dumbbell above your chest.
Depending whether this exercise is intended to focus your lats or chest (it will be indicated in your workout program) try and focus on using that specific muscle.
For example, if it is for chest - when you're bringing the weight back to your starting position focus on squeezing your chest muscles.
If it's for your lats, then focus on moving the weight with your lats.