How to Run a One-Shot in One Session
I commonly see GMs online lamenting "whenever I run a one-shot it ends up taking 2-5 sessions instead". After running 2.5-hr open table sessions at my university club for a year I am at this point reliably able to run a one-shot without running over time. Here's my basic method:
Node-Based Design
Obligatory Alexandrian link. Basically just break up your prep into discrete chunks, encounters, scenes, etc. It can be helpful especially to note down the default challenge type of each node, like Combat, Traversal, or Social. It's okay for a one-shot to be linear, but node-based design is still very helpful for our purposes here.
A simple node structure based on a real one-shot I ran.
If you're worried about not having enough content, you can also prepare some "floating" encounters or scenes that are completely optional, but that you can drop in to the gameplay if you feel like the players are going too fast. Usually players are going too slow, though, more on that later.
Keep An Eye on the Clock
With the one-shot broken up into nodes, you can roughly guesstimate an amount of table time that each scene should take up. A short investigation in a single location will probably take less time than an elaborate combat against a boss and minions. Your estimates are unlikely to be entirely correct, but that's alright, you just need something to steer by. Set up your nodes to take around the same amount of total time as your session timeslot.
You can never really predict how long any given group will spend/waste on a given scene, but with experience you can get a decent sense of an expected time based on scene type and complexity.
Also keep in mind that transitions between scenes themselves take some extra time, especially if players are given a choice in direction to go.
What's important that is you keep the key scenes in mind and how much time you want to have left when the players reach them. In this example, the Ale Oozes and the Necromancer combats are the key scenes. The Oozes happen right at the start, so there's not much danger of running out of time, but the Necromancer is your climactic conclusion and will likely take 45 minutes to an hour of table time. This means that while you're running the preceding scenes, you need to keep that 1 hour budget in mind.
The goal here is to be able to run that entire Necromancer combat without going over time. If, during earlier scenes, you start to notice your time budget slipping, begin deploying time-saving measures:
Keep Players Moving
This one is pretty basic table management stuff, but you need to be more aggressive about it to keep a one-shot on time. If the players get stuck going in circles, confused about the rules, or talking about something unrelated to the game, you need to just step in and keep the game moving forward. Point to a PC and tell them they notice some key information (no roll required), make a ruling instead of looking up the rules, or cut into the conversation and ask what the players want to do in-game.
Especially in combat, you want to be actively pushing players to take their turns without waffling. Alert players when they're up next, and quickly recap the situation when a player doesn't seem to know what to do. Be willing to break the turn order and say "okay, we'll come back to you, next player, what do you do?" if a player is really hemming and hawwing over their turn.
Do not ever lock progression forward in the session behind a PC rolling a success on a specific skill roll.
Cut Fluff
While grand narration can be a great boon to the game, you're on the clock here. Don't spend lots of time describing every attack roll or saving throw. It's okay to just say "the zombie hits you for 4 damage", especially after the first attack or two. You don't want to reduce the game down into being a boring description of rules outcomes, so strike your own balance with this.
Don't let scenes overstay their welcome once their purpose has been served. You can speed up combat by having enemies flee or fudging their HP downwards. Traversal and Investigations can be sped up by reducing the number of checks or their difficulty. Social obstacles can be sped up by having NPCs lose patience or get interrupted by other, more helpful NPCs.
Cut Content
Your ultimate weapon. This is where breaking up the session into nodes comes in handy. If you're an hour and a half into the session and the players are just now finishing up the first combat, it's time to take an axe to your prep. With your key scenes in mind, remove non-essential upcoming nodes from the session. Skip random encounters, turn investigations into instant discoveries, collapse decision points into linear paths.
Oops, the table really got into roleplay in the intro and then got bogged down in basic rules on the combat, time to simplify!
Sometimes its not so drastic, just removing a single encounter or streamlining a node connection can cut 10-30 minutes from your runtime. Remember that the goal is to reach and complete the key scenes, to run a fun self-contained session within the time allotted, so don't be at all precious about anything you prepared.
Other than these time-saving methods, running a one-shot is not so different from running any other session. I find it very satisfying to conclude a climactic boss fight and end the session right at the 3 hour mark, wrapping up an exciting night of D&D, rather than bogging down for another hour or trying to figure out scheduling another session to finish up the adventure. You might even find the exercise helpful in making your others sessions punchier.