It is not a good idea to have a sample letter. The letters of support from friends and family need each to be written from the point of view of that individual. They didn't all have the same experience of meeting the partner and seeing how s/he became part of the family/friendship group, and knowing that the couple then developed a live-in relationship. If they were all the same, they wouldn't BE genuine, and they wouldn't SOUND genuine, so they wouldn't satisfy INZ.
Each person needs to state who they are in relation to whichever one of the couple they knew originally (e.g. father, neighbour, childhood friend, family doctor, or whatever). Then they need to tell the story. Their friend first started to talk about the nice man/woman s/he'd started to go out with, having met at (wherever), roundabout (whatever month, whatever year). They first actually met the partner (when, where, under what circumstances - e.g. by chance out shopping, invited out for a meal, at a family barbecue, etc., etc.). First impressions of him/her, and noticing how the person they know already was serious about him/her. Gradually getting to know him/her better - any common ground like liking the same music, or watching/playing sport, or whatever. Tell about him/her now getting to be included as a matter of course with the partner in any family/friendship group activities. Tell when they knew the couple had moved in together, and that they were pleased to hear that. Tell about any special occasions both the couple were at together, or holiday trips with some significance. (When I wrote this letter about my son's wife, I said how she attended the day-long party for my mother's 80th birthday, along with relatives and family friends going back to Mother's childhood, and also how the couple went touring France together, making a special detour to stay with my son's godmother, who is a very significant person for him.) Tell about other occasions of visiting the couple's home, or picking one of them up from there and seeing the other. If theirs is a professional relationship (e.g. the doctor), they could state that the partner then became a patient of the practice, or whatever happened.