You have two elbows. The one on the bow side needs to be more or less vertical, so that if you were to bend it, your hand would come in to your chest rather than up toward your head. Keep it straight but unlocked.
The one on the string side is more problematic. As you have noticed, it is difficult to be aware of the back muscles directly, so we use indirect clues. One of them is to pretend you are closing a door with your bowstring elbow. If that helps, use it.