How do you write an Affidavit for insurance purposes?

It will probably need to be notarized. If you are a member of a credit union, you can have it notarized there for free.

As far as the content of the affidavit, I would ask the insurance company to explain what they want you to write. In other Words, do they want you to write a detailed recount of a claim? Do they want you to write from start to finish, what was the initial cause, how you discovered the damage, what you did when you discovered the damage, etc?