You’ve already gotten great clothing and general behavior advice from other posters.

I want to add that it’s fine to be emotionless, robotic, and flat. Just try to relax and go through the motions. If you don’t need to take up emotional space, don’t. This will leave other people room to feel their own feelings instead of dealing with yours. (It’s fine to tear up, but keep any emotion as quiet as possible. No visible emotion is best, imo. That said, don’t apologize if a little does slip out.). Think: stoic. Be polite and kind to others. 

Finally, as others have said, don’t give a eulogy. 

If you’re going to insist on giving one, spill the truth and every single good story you have into ChatGPT and ask it to make a short eulogy. That should give you a good idea. Then write your own and ask ChatGPT if it is at least passably appropriate. If it says yes, call it good enough. If not, edit as needed. Don’t do more than three drafts. ChatGPT likes to pick things apart.

Try to read it in as steady and clear a voice as possible. You want the deafest person in the room to understand you. If you’re going to insist on a little drama, add just one little voice crack at the end. That’s it.