Between all the isolation tricep exercises, the most popular is the french press.
It can be performed with a straight barbell and dumbbells, but the EZ bar is the one that most people prefer because it lets you preform a movement that comes more natural to your elbow and wrist joints.
French Press Details
PRIMARY MUSCLE: Triceps
SECONDARY MUSCLES: Forearms
EQUIPMENT: EZ Curl Bar
Why are forearms always somehow involved? Well it's just a mechanical thing, just try holding a dumbbell and you feel your forearms tight...
INITIAL POSITION: From a standing position, grasp an EZ bar loaded with the desired weight and raise it above your head extending your arms. Palms face forward. Don't bend your wrists. Slightly bend your knees to find a stable position. Keep your core tight.
MOVEMENT: Lower the bar behind your head, slowly, focusing on the movement. Then lift it again to the initial position. Keep your elbows high in the same position, don't swing your arms. Lower and lift the bar in a semicircular arc behind your head.
BREATHING: Inhale while you lower the bar, exhale while you lift it.
TIPS and ERRORS: Prefer the EZ bar when you perform the french press as it gives more comfort relieving pain in the wrists and lessening torque in the elbows. Don't move your elbows, don't let them move behind while your lower the bar and don't bring them forward while your lift it.
The most common variations of this tricep exercise perform the movement in a seated or lying position, or use a straight barbell or dumbbells.
Seated position: it is exactly the same of the standing position but it provides more balance and stability for obvious reasons. Especially if you use a back support.
Lying position: this one deserves a little bit more explanation. It's not as easy as just "rotating" the standing position of 90 degrees. Once you lie on a flat bench your elbows point straight up (arms are vertical). You lower the bar behind your head keeping your elbows pointing up and the lift the bar again. You notice that when you lower the bar you can't actually bring it behind your head without moving your elbows. Just go down as much as you can (probably just behind your forehead).
Dumbbells: just repeat the exercises described above with dumbbells. Try to keep the same orientation of your palms that you have when you perform the french press with EZ bar. Also with dumbbells you can do the exercise standing, sitting or lying on a flat bench.