You will write it just as if you were writing a letter to someone and descrbing what a wonderful friend that have been to you. Start by describing where you
met and how your friend have helped and comforted you through the years. There is no need to dwell on unpleasant topics as the person is gone and unable to defend themselves. Describe unselfish acts they have completed and other good works that may have been done unanimously. Say kind words that will offer some cheering to your friend's family. If your friend was a member of any organization or won awards this will be the time to let them have their moment. Above all stress how the world was a better place to have them in for a brief moment and if your friend had a special song or poem try and let that be the final thought.