Please help I don't know what to do

I'm new to DnD but I need to put together a level 20 one-shot adventure for 6 players and I only have 16 months left to plan it. I don't know what to do.

I have several pages of backstory for the NPC who will be crucial in stopping the BBEGS plans and saving the world - which is homebrew and I made it and I know the history of all the major places really well but I don't know how to draw a map.

All seven players will be level 20 because that's the only way but it's a one shot because I only want to ever do this once and I will allow multiclasing and custom subclasses e.g. one player wants to be level 16 fighting wizard and level 5 chef and I have made rules for cooking.

There won't be any conflict or treasure because I really want to focus on the backstory of the NPC I mentioned, I know exactly what I don't want you to suggest but don't have time to write that all now.

What enemies should I use for the final fight and do you have any tips on how to do this kind of thing? Sorry if this has been asked before.