Contractile failure is the main trigger to stimulate muscle growth. That means you go until can't get another full rep with good form.
The key in this variation of rest-pause training is hitting failure at several points in the same set. Basically, one "set" may be two minutes long with little 10-second breaks taken within the set. "Death by" means that you continue until you can't do any more. Here's an example.
Hamstring Curl, Death-By Rest-Pause
Start with a weight you can do for 6-8 reps. The number you get isn't important, this is just to help you pick the best weight to start with.
Go to failure. The last rep should be tough, but you should be able to do the full rep in good form. Don't try to do half of a ninth rep and then fail. Remember, contractile failure means completing a FULL rep and not being able to do another full rep.
Rest for 10 seconds.
Using the same weight resume the set and once again go to failure.
Continue the pattern: reps to failure, 10 second pause, reps to failure, 10 second pause... until you reach a point where you know you can't possibly get one more full rep.
Continue doing the same process until you can only perform a single complete rep in a mini-set, where you know that getting a second one would be impossible.
Since these sets are amazingly powerful but also traumatic on the body, only do one or maybe two sets per exercise. This method is ideal for isolation lifts.