The past perfect is useful for putting past events in order.
You are most likely to use it in your letter to explain previous medical or social history.
When describing two events which are both in the past compared to the time of speaking or writing, then the event furthest in the past is described using the past perfect. The event more recently in the past is described using simple past.
To form the past perfect you use had + past participle
Stephanie had told her mother she felt sick the morning she fainted.
Mrs Le had taken the medication as prescribed but it did not alleviate her pain.