The past perfect is formed according to the formula: conjugated modal verb in imperfect + perfect participle

imperfect of to have   + perfect participle   I had you had he, she, it had we had you had they had + eaten read bought written seen gone

The choice of the modal verb is exactly the same like with the present perfect - transitive verbs (those ones which have a direct object) use the verb haben, intransitive (the ones without direct objects) use the verb sein. And the perfect participle is the same as described before.

imperfect of verb haben + perfect participle   ich hatte du hattest er/sie/es hatte wir hatten ihr hattet sie hatten gegessen gelesen gekauft geschrieben gesehen

Examples   Ich hatte einen Brief geschrieben. = I had written a letter.   Wir hatten Brot gekauft. = We had bought some bread.   Ihr hattet einen Mann gesehen. = You had seen a man.

Imperfekt del verb sein + participio perfecto   ich war du warst er/sie/es war wir waren ihr wart sie waren gegangen geschwommen   gelaufen gefahren geflogen

Examples   Ich war nach Amerika geflogen. = I had gone to America.     Wir waren nach Hause gelaufen. = We had gone home.     Das Auto war kaputt gegangen und deshalb nahmen wir den Zug. = The car had broken down and therefore we went by train.