Pre-ramble
My group plays every other week on average, so keeping an adventure short enough to be completed within two/three months at most is important to me. It makes it easier for everyone to remember what the goal is and relevant details to the current quest! I also prefer a more open ended approach - I don't wanna know what's going to happen next, so I don't really enjoy an adventure that's too linear! ^w^
I have a list of some rough guidelines that help me build adventures. You can string multiple of these adventures together to run a campaign, or just create ones that stand alone. To create an adventure, regardless of the type (dungeon delve, stealth mission, political intrigue, etc) I use these five stages to flesh out the general idea:
Steps:
1. The Goal
2. The Location
3. The Time Limit
4. The Bad Ending
5. The Antagonist
The Goal is what the party needs to accomplish in order to succeed or complete the adventure. It can be as simple as "go recover the SoulGem", or as complex as "stop these two nations from going to war". Sometimes there are multiple goals, and some of them may be optional; either way it should be clear to the party what they are trying to achieve during this adventure. Note, you don't have to decide how the goal is supposed to be solved - leave that to the party!
This one is very straight forward - where is the Goal located? Your party may not start out there, but you should know where the SoulGem or two nations are, as well as some basic information about it (climate, any nearby locations of interest the party may go to, important NPCs, etc).
If you can communicate to the party the Goal and where it is Located within 15 mins of your session you're absolutely crushing it!
The Time Limit helps keep both the DM and party on track, with a clear cut off point for when the Goal needs to be accomplished. This limit could be as short as a few hours, or as long as a year (I prefer a shorter limit but it's to taste); the idea is to make it clear when the players have to be done. It helps to avoid that weird feeling when you've spent three in-game weeks and several real-world months goofing around, and meanwhile the Villain hasn't made any progress in their goals somehow? When the party runs out of time, the bad guys win, and this is where we can get stakes, excitement, and drama!
What happens if our party doesn't manage to complete their goal? What if they roll really bad, make a terrible plan, or run out of time? Here we establish the concrete consequences of their actions, and preferably let them know what could happen if they don't act now. By planning a bad ending, you both have to figure out what the Villain wants (if you use a villain), and what the world will look like if the party fails in their goal. You can end up with some very interesting consequences, and the party can feel like there are really stakes for failure! If you end up with the Bad Ending, it will often give you a clear and strong hook into the next adventure - cleaning up the party's mess!
Lastly we need to figure out who our antagonist(s) is/are. This step is optional, not every adventure will need one, but they can help add interesting factions and NPCs to your world if you want some more roleplaying opportunities. The Antagonist is any person or faction who's motives conflict with our party's goal - it can be the goblin tribe in the dungeon, or a rival group of adventurers, or the tyrannical ogre bossing the goblins around. Their role is to be a source of drama and get in your party's way. If the Antagonist wins, it could lead to the Bad Ending, or just further their own plans and lead to new problems/adventures down the line.
With these steps you can plan out your own cool adventures! I hope this is helpful! ^w^