When do I build the trapassato prossimo with “avere” – and when do I use “esse”?

You already came across these rules, so a quick revision here will be sufficient.
When putting a transitive verb into the trapassato prossimo, you use the auxiliary verb “avere”.
When built with “avere” the past participle of the main verb doesn’t change according to the subject of the sentence.

💬 Examples:
Io avevo mangiato una pizza. – I had eaten a pizza.
 
The auxiliary verb “essere” is used with intransitive verbs and reflexive verbs.
Here, the past participle changes.

💬 Examples:
Ieri sera ero andata (f) al cinema. – Last night I (f) had gone to the cinema.
Ieri ero andato (m) al cinema. – Yesterday I (m) had gone to the cinema.

Now, let’s focus on the usage of this tense.
The trapassato prossimo is used when talking about an event or action that happened before another event in the past.

💬 Examples:
🇮🇹 Dopo che aveva pranzato, Paolo dormiva per due ore.
🇬🇧 After Paolo had lunch, he slept for two hours.
🇮🇹 Eravamo partiti prima che cominciasse a piovere.
🇬🇧 We had left before it started to rain.

➠ Future Tense(s) in Italian