Helping friend with their AOS. Affidavit letter to show support

[annajoanna]

My friend is in the process of getting her citizenship and will be applying soon. She has been approved for the conditional 2yr green card.
She has asked me to write a letter proving that the relationship is actual and in good faith.

Is there a format I should follow? or any links to examples of such letter?
I dont what to do anything that will hurt them

Thank you