How does the gender reveal work?

Hey mamas I'm not that far along, only 6 weeks & 4 days. My first appointment is next week Tuesday but I saw a post and it got the best of my curiosity. How exactly does the gender reveal thingy work ? Like, I want my fiancé and I to find out together but in the privacy of our home but I want us to find out in a cake. How would that work? Would I have the doctor write down the sex in a paper, put it in an envelope, take it to publix and be like "Yes, I'd like a gender reveal cake, but I want the inside to be either pink or blue based on the sex written in this envelope" like would I sound crazy ? 😭😂 any help please?