If you knew the deceased, please know that your words are a gift of memory. They will offer the recipient a small unknown perspective of your way of seeing that person, which is unique. Share a story, a moment, a memory, a realization—it doesn't matter what. It just is something you know, thought, experienced about that person.
In sharing it, you make that person come alive. You continue to expand and grow the recipient's knowledge of that person—something they may have felt had ended with their loss. You give the gift of the knowledge that you too are a repository of memories that live on. That a life was valued, had impact, was appreciated.
It does not need to be lengthy—or even positive!