How to create an adventure.
Step One: Ask Two Questions
Every adventure begins with just two sentences: What's the problem? How is it resolved? Write one sentence for each. That's your skeleton.
Step Two: Add Flesh
With your problem and resolution in hand, start adding the muscles and skin: encounters, NPCs, traps, maps, and locations. Some detail should connect back to either the problem or the resolution, but always making the journey challenging.
Example:
Problem: A farmer wears a cursed amulet and becomes possessed, raising the undead. Resolution: The adventurers must return the amulet to its resting place. From that seed, you already have a quest: confront the farmer, deal with undead, recover the amulet, travel to a desert ruin, and return it to break the curse.
Step Three: Scale to the Party
Add details suited to the group's level - weak skeletons for beginners, or giant desert scorpions and hordes for veterans.
That's It.
Two questions, two sentences, and you have the foundation of an adventure.
From there, go deeper - add maps, encounters, NPCs, monsters, traps