How do I explain when to use "had had" vs. "had"?
I'm not a teacher.
Marcus had an apple last friday.
Before Marcus had an apple (last friday) he had had a banana.
When you want to say that an action has happened before another action in the past you should use the "past perfect". In this case "had had".
When you want to say that something has happened the past you should use the present perfect or simple past.
In this case "had". But remember to differ between present perfect and simple past.
That's how it was described by my teacher....
For more information use google typing "past perfect".