My family and friends variously used handwritten letters, typed letters signed by them, and emails. It doesn't seem to matter, and only if you are applying as a common-law couple do at least two of those need to be notarized affidavits, otherwise they don't have to be notarized.
The letters basically included:
1) I am related to/have known sponsor and/or applicant since X date. (To establish who they are).
2) I am asking that you approve applicant's request for permanent residence in Canada so that (they explain what they know of our marriage and our plans for life together and why we should be approved).
They all wrote their own letters, we just asked them to talk about how long they've known us, what they know about our relationship and why they believe we should be permitted to live together in Canada. We didn't give them any other guidelines. All the letters were very individual, and I think that is better than a bunch of letters that looked like you gave them all the same template and they just filled in their names.