what is required-Letter from friends stayed with during FLRm

[Burunt]

Hi,

I am applying for Set(m). Currently on FLRm.
For a few months me and my partner stayed with a friend who owns their home after we moved house and were looking for a new home to lease.
We did not pay rent, so did not have a tenancy agreement or have our names on council tax or bills. We did not have any letters sent there to us or registered with anywhere while we lived there. How does this affect our cohabitation?

What does this letter need to state and does it need to be certified or anything. If so can you please explain.
Is there a template for a letter like this?

Thank You