By far my favorite tricep exercise!
Step 1 sit comfortably witch straight back (can also be done standing)
step 2 temporarily place the dumbbell on your legs or shoulder
step 3 make sure your dumbbell is behind your head with your arms straight
step 4 lower your dumbbell with your arms closed as close as possible to your chin
Step 5 repeat this
I do this exercise 4 reps 12-15 sets