Tricep Extensions Demostration

I always struggle stabilizing the weight and end up not isolating the triceps and put stress on my shoulders. Any tips?

So do you lack range of motion in your shoulders?

I have bad shoulders due to torn rotatory cuffs. Parents didn't have the money to get them fixed in high school when I blew them out so the shoulder has kind of healed that way. I have a hard time isolating due to I want to compensate for the shoulders. Is there a better way to isolate to build strength and mass?