I went to the wikihow site and created a sample. Probably it will be something similar. Since you already know that affidavit is merely a written statement confirmed by oath or affirmation, for use as evidence.

The attached is only intended as a sample. You may refer to your immigration attorney for more details.
This is just to provide an example, that preparing an Affidavit could be a simple process.

Probably one can create one affidavit each for each of their parent or relative.

Source:
http://www.wikihow.com/Write-an-Affidavit