This must be a real speech, using real people, real stories, and an imagined real audience. The writing will include getting everyone's attention, perhaps using the fork on the glass technique, which will be included in your writing and the video performances. Introduce yourself to the wedding guests. Keep the focus on the bride and groom, not you. You could do it in threes, beginning (past story) which might include how you know them, when you met them, middle (present story) this might include your present relationship, end (future wish) what you wish for them and you may want to use a quote. You may choose to use a story, a story that exemplifies how great one or both of them are.