1) Holding a dumbbell in hand, sit on a flat bench and rest the dumbbell on one knee. Preferably, use a bench that has a short backrest. This will help keep your back straight.
2) Grasp the base of the dumbbell - the part where the bar of the dumbbell begins, under the plate. Use one hand over the other.
3) Kick the dumbbell up with your knee and rest it on your shoulder before lifting it above your head. Or, if you are confident, lift it directly above your head.
The dumbbell needs to be behind your head so that when you lower it your elbows go beyond 90°.
4) Keep your elbows relatively close to your head and your upper arms vertical.
5) Inhale as you slowly lower the dumbbell. Make sure that you feel a good stretch in triceps - your elbows should go beyond 90°.
Make sure that you are moving only your forearms and your elbow joint to move the weight. Try and use your triceps as much as possible and avoid using your shoulders.
6) Exhale as you lift the dumbbell back to your starting position. Do not fully extend your elbows fully - keep a slight bend.