How do I take the back by getting behind the elbows?

If you want to take the back, focus on getting behind the elbows. The majority of the time, the only thing stopping you from taking someone's back is their arm. If they have an underhook, you cannot take their back because their arm is in the way. But if you get behind their elbow, their arm naturally is not in the way anymore. An arm drag is a great example of this principle.