Hold the dumbbells over the middle of your chest (just below the nipple), so the weights touch one another (your hands and weights should be in the same position as if you were lifting a barbell), pointing your knuckles towards the ceiling. Push your shoulders down so that they don’t come off the bench and push down on your heels.
Lower the dumbbells keeping them just below the nipple until they are just off the chest and without pausing push them straight back up again, keeping your shoulders and head on the bench at all times..
The closer your hands are together (ideally dumbbells touching one another at all times), the more you will isolate your triceps. Try and keep your elbows as close to the side of your body as possible.