Here is a quick exercise how-to on Tricep Extensions.
Set-Up: Stand tall with your feet shoulder distance apart and a slight bend in your knees. Contract your abs and tuck your tail bone under, to brace your torso and support your lower back. Grasp one end of a dumbbell and carefully lift it up overhead. Your bent elbows should point directly upwards with the dumbbell behind you and your palms facing upwards. In this position, check that your abs are still contracted to keep your back from arching.
Exercise: Keep your elbows tight to your ears, and slowing extend your elbows so that the dumbbell is lifted up over your head. Your palms should remain facing upwards and the dumbbell mostly vertical. Lift the dumbbell as high as possible while keeping your shoulders pressed down. Do not lock your elbows at the end of the motion. From the top position, slowly bend your elbows, lowering the dumbbell back down behind your head with control. Your elbows should remain fixed beside your ears and upper arms vertical throughout the entire motion. Exhale as your lift the dumbbell up, and inhale as you lower it back down.
General Tip: Avoid the common mistake of using your body’s momentum to help lift the dumbbell, by consciously contracting your core muscles throughout the entire exercise.