Tricep Kickbacks
Tricep Kickbacks are good isolation exercises which usually involves a single dumbell and a flat bench, although there are a few variations. Form is difficult to maintain on this specific exercise but keeping the elbow in and the tricep parallel to the floor should ensure good form.
I recommend using two dumbells, one at 40% of your 1 rep max and the other at 60%
Below is an example of this workout-
SET 1
8 Reps @ 60%
12 Reps @ 40%
SET 2
8 Reps @ 60%
12 Reps @ 40%
SET3
8 Reps @ 60%
12 Reps @ 40%
You will feel the 12 reps @ 40% will burn and remeber to squeeze the dumbell at the top.