Writing for DnD adventures can be stressful and difficult, but it doesn't have to be. The difficulty of regular writing tends to be with creating engaging plots, relatable and interesting characters, and setting it in a fun world. Some of these challenges remain for DnD writing, some are more difficult, and some are missing altogether. Let's run through some tips and tricks.
1. A Strong Call to Action
Players complain all the time about being railroaded in DnD. This has some value, no player wants to be told exactly what to do since that takes away agency, but they still need direction. Something to do. A call to action is simply a statement or enticement to complete a task. Motivation can come from gold, adventure, love; really anything that would inspire a person in normal life. What a call to action does is identify a motivation and demand for it to be acted upon. Identify a task for your players to accomplish without telling them how to do it.
2. Have at Least One Charismatic NPC
Story needs changing characters. Characters that change are satisfying and give a reason for the reader/listener/viewer/player to have spent their time engaged with the story. Without character change, a story is just a series of events. But, since your players are going to be the protagonists of your story, it is easiest to make your antagonist the one that changes. That doesn't mean it has to be a straight "I was bad but now I'm good" type of change, but the NPCs that butt heads with your PCs should change in some way.
3. Make a Location and Boss Fight
Your location can either be at the beginning or end of your oneshot. A location is a well fleshed out set piece that your adventure spends most of its time in. This, from a story perspective, acts as its primary setting. If you're writing a mystery or roleplay heavy oneshot, you probably want to put your more detailed location at the beginning of the adventure. The primary reason for this, is that oneshots tend to end with combat, and that combat should include your boss. A boss is the most threatening enemy. Threats come in a variety of forms, but since oneshots tend to include PCs that have less defined character traits than long-term campaign PCs, that threat should probably mostly be physical. If a boss makes a PC question the nature of their being it can often cause the player to simply change the very nature of that PC. Moral questions are fun, but if you're asking a moral question in a oneshot, try to make it an extreme question, with a relatively easy answer. If your boss is physically threatening, simply make them a difficult enemy to kill.
4. Provide a Clear Ending
There's nothing less satisfying than a cliff-hanger. The only time you should employ a cliff-hanger in DnD is if you know for sure you'll be playing with the same group again. Or, arguably, if you're trying to entice a group to keep playing, but that's a bit of a cheap tactic. A oneshot is like a short story, and a short story must have an obvious ending. It doesn't have to be a happy ending, the PCs might all die or achieve none of their goals, or fall victim to poor decision making and fail to complete the call to action. But when the oneshot is over, the players should know it is over. Often a great way to do this is to just have a little wrap up text, almost a "Where Are They Now" for each of the characters, to show the consequences of their actions and how it all panned out in the long run.
Storytelling is hard. The beauty of DnD is that you as the DM are not the only storyteller, you don't have to decide what all the characters do because most of them are under the control of the players. That being said, there are some things you should prepare to ensure the oneshot you create is compelling and narratively satisfying. Those things are the Call to Action, the Location, the Boss, and the Ending.