Laying on a bench hold a dumbbell with both hands and take behind your head and bring back to your chest.
Take hold of a 10kg weight and lie down flat on a bench and hold the dumbbell with both hands (when you put a dumbbell on its side you have the weight on the ground and the weight in the air take hold of the underneath of the weight in the air, overlap both hand so you have a hole in the middle and resting on your hand should be the plates of one side of the dumbbell) hold the weight on your chest and extend your arms all the way back so they are behind your head then bring them back into your chest. Repeat this nice and slow so you get maximum benefit.
If you are starting then you could do:
1st set 10kg 10 reps
2nd set 12kg 8 reps
3rd set 12kg 8reps
4th set 10 kg 12-15 reps
Then next week move up to say 14kg and start at 12 kg.
[image from workoutlabs.com]