The question is better suited to misc.legal or misc.legal.moderated,
but I'll answer it here. I have a private mortgage. The term "Seller
financing" on a house for sale means the seller will consider giving
the buyer a private mortgage. It can be a good deal for both parties.

You need 2 forms, Mortgage (aka Security Deed) and Promissory Note.
Using FannieMae / Freddie Mac Uniform Instrument (so called FNMA)
versions of these will make it easy for all parties; a uniform
mortgage is, well, uniform. You can get the forms an old-fashioned
stationery store, the kind that sells forms! Or online: there are
many sources. I got my forms (FNMA) online from uslegalforms.com .

A mortgage is a mortgage is a mortgage, for legal and tax purposes.
5% is well within bounds of market rate and thus does not qualify as
a gift.

Una