The question itself is flawed because the triangle choke is a technique that requires so much adjusting depending on how your opponent reacts--whether he postures up or stacks you, which way he turns, where his trapped arm is, where his free arm is, etc. So the best way to drill a triangle is not one way, but many different ways. You have to be able to set it up, lock it on, and finish it, no matter what your opponent does to defend.
Find a training partner who will give you a little bit of resistance while you try to triangle him and he tries various different escapes.