How to form the passive
Active: subject + verb + object.
Passive: object becomes subject + form of 'to be' + past participle.
Examples:
Active: She wrote the letter. → Passive: The letter was written (by her).
Active: They build cars here. → Passive: Cars are built here.
Active: He will deliver the package. → Passive: The package will be delivered.
Use 'by + agent' only when you want to mention who did it. Often, you do not.