What has to happen to use être instead of avoir for the formation of passé composé?

To add to the answer besides mine, you have to have noun-verb tense agreement, which means if the verb is feminine, an e is added to the end of the verb which is conjugated in the passé composé (NOT the helping verb). If it is plural, an s is added at the end of that same verb. Feminine and plural adds an es at the end of that verb. Hope this is helpful to you