There are a lot of good examples around this forum, just search through the posts.
We wrote an account each of how we met and how our relationship progressed, included letters of support from family & friends, wrote a relationship timeline, included photos and individual sections with proof of our engagement, wedding, honeymoon and relationship in general... maybe I'm missing some things out, but this was generally what we had. Of course, some of this was filed under "supporting documents" but we referred to some of them in our sections in IMM 5540 and 5490.
Tips:
- Write a timeline of important relationship events, so the IO can clearly piece together where certain photos or events happened
- Both write an account of your relationship and how you feel about one another, annotate with photos if you think it fits
- Write important dates & events, like when you met, when you met each others friends & families, any challenges you faced in your relationship or whatever... etc etc..