From my latest publication (A Minimalist Fantasy Companion). Don't worry. The entire text is released under a CC license.

This is the system that I use for creating quick one-shot scenarios. An Autumn Feast is the sample scenario included with A Minimalist Fantasy Affair (which you can get as a PWYW - even free - download
here).
Step One: The Setup
Write a short paragraph or two detailing how the player characters get hooked into the adventure.For example, maybe they are hired by a third party, invited to an event (as was the case in An Autumn Feast), summoned to a meeting by an old mutual friend. And so on.
Step Two: Behind the Scenes
Write a short paragraph or two detailing what is going on behind the scenes (things that the player characters are not privy to prior to the start of play). This is the adventure’s underlying plot and it informs Step Three (below). In An Autumn Feast, the local Lord who invited the PCs to a feast had nefarious (secret) plans to summon an interdimensional deity that he worshiped.
Step Three: How Things Unfold
Write a short paragraph or three detailing how the adventure unfolds (or how it may unfold)when the player characters interact with the plot as defined in Step Two (above). This is where the setup and the behind the scenes info intersect byway of the PCs’ actions.Make notes about possible outcomes, both good and bad (don’t worry about anticipating them all; you can’t). In An Autumn Feast, I listed what could happen if the PCs intervene in the local Lord’s plans. Admittedly, I forgot to explicitly mention what happens if the PCs can’t intervene,don’t intervene, or fail to stop the local Lord and his lackeys (but, using this basic adventure structure, it’s pretty clear what those outcomes will be).