Our application did not require family and friends to write a narrative.
My husband and I each wrote a narrative. We each wrote one page, one paragraph of how we met and the rest talked about a few things that happened in the years since. I mentioned what happened on a few trips, for example when he proposed and when we got married. I also included that I learned to watch for deer on the road because I ran over one that someone else already hit. Then I mentioned that my husband bbq'd steaks for us when it was -40c and he had to use propane torch to melt ice off knobs.
He wrote about a couple of things like how we each accept each other exactly as we are and how when he proposed he didn't have a ring so he made me one and a year later bought me a ring. He mentioned meeting all my friends and coworkers during his first visit. He wrote about why he loved me and how his love for me grew.
I suggest you just start writing as if you were telling a friend about how you met and fell in love. The first sentence was the hardest for both of us, we didn't know where to start. But once we did, the rest was easy.