One floor at a time. You don't need the entire dungeon to start. Mark rooms with numbers or a combo of letters and numbers and write what's inside in your notes.
It's for a one-shot, dungeons/rooms are already designed, there's a small cabin with a basement, its more of an exploration encounter, lots of traps. And then I have a small three room dungeon, a spider tunnel, basically multiple tunnels, leading to one small, medium, and large room.
Struggling with writing out the dm prep for it. Should I just write the traps, and monsters in each room, what each does/numbers, loot and etc. Since I already have all the info in the dm map design, and where each thing is. I don't want to over prep
One floor at a time. You don't need the entire dungeon to start. Mark rooms with numbers or a combo of letters and numbers and write what's inside in your notes.
I’ve been having the same issue lately, and what I decided on was just not including pointless rooms/areas. If there’s not that happens in there, why am I wasting my time and my players times? So each room needs to have a purpose. Doesn’t matter if the purpose is a fight, some loot, whatever it may be, it just can’t be “you walk into this room and you see absolutely nothing worthwhile”.
Use index cards with separate challenges, as they believe room to run run over a new card.
Prep some. Make a lot up on the fly
I mean... Monster stat blocks and loot seems the base of the prep...
TLDR: Focus on the broad strokes, not on the whole thing all at once. You can fill in the gaps later. Focus on the core points of interest first. What rooms will the players spend the most time on? Which rooms will they pass by as intersections? One-Shots run between 2 to 10 hours most of the time, limit the amount of things to interact with for the essentials for the plot to progress. Cut out encounters which aren't necessary, with the encounters as a whole referring to not only fights but also skill challenges and things like it. Once you have the core of the dungeon, then you can start filling in with other smaller details. This can be done as part of your prep, or done on the fly, given it matters less. Important rooms are 2 to 3 paragraphs of text at most, while unimportant should be just a paragraph, if not limited to two sentences. I personally divide rooms in chapters in my written preparation, so I'll have text dedicated for the Entrance, a Hall, a Hidden Passage, etc. That way, I just have to look at the title to know what to read, instead of reading a huge chunk of text to find what I'm looking for. You can also put codes on each room for you to look what you're looking for without the guess work of remembering what's what (1A for the Entrance, 2A for the Hall, 1B for the upstairs, and so on.).
Make it up as you go along. No plan survives contact with the enemy, and no DnD session prep survives the interaction with the players. Write the barest level of concept of what you want to happen down, and then wing it for the rest. If you plan it, they'll either break it or avoid it.
Think about any of the Indiana Jones films, national treasure movies, or any like that.... Not every room has to have something exciting in it. Every nook and cranny needs to be explored. I would think about the point of the dungeon first and why the party is even there. Is t just a small cabin or is it the entrance to a bigger lair for some creatures attacking a local village or a base for some cult, or a group of Bandits? Is it an abandoned, why was it abandoned? Did they just stumble onto it? If there's no reason for them being there, then there's likely nothing to keep them interested in exploring it. Once you have the purpose for it, you can start tackling things like what kind of encounters could there be and how many? What rooms would be important to have and what would just be vacant/dead space? Lastly and most importantly, what does it lend to the story you're trying to tell? If it's just there for them to have something to do, then whats the point of having it at all? Does it lead to something else, a bigger element of the one-shot? Are they going to find something that could be needed later? Or is it just some cabin in the woods that hasn't been used since the last owner died? Prepping a dungeon is significantly easier to do when you have a purpose in mind for it.
It's a one shot. You can prep it all in advance and run the party through on rails. Are you homebrewing elaborate traps and puzzles? Then you can just make notes " Arrow trap: first character to touch the door must make DC15 Dex saving throw or take 1d8 piercing.". Treasure doesn't matter unless it's needed for the quest.
Now, King Thamus of Egypt doesn't want you to know this, but by representing language and thoughts as marks on a surface, you can remember things without having to keep it all in your head. Also, it's a three room dungeon. You can probably keep it all in your head.
Prep little. Focus on ideas. Work on the opening scene. Know some information the party can learn, if they actually ever take a break from stabbing things. Develop a cool location (not too much detail focus on cool). Have some key foes and NPCs. Have a short list of treasure. The dice will tell the story.