There are a lot of great tools you can use to design your maps. With that said, the first thing I always do is sketch the map on a notepad with a pencil. The map can be good, but you should have a general idea of the world you want to create. After I have drafted the map, I will list the types of encounters I would like the PCs to experience. This gives me a firm idea of what I want the whole adventure to look and feel like.
Next, I open my favorite dungeon design software: DungeonDraft. This is not a promotion for DungeonDraft. It is just the one I use. The map will often take me 2-3 days to draw as I will add details used in encounters. For instance, as I was creating the map for The Mad Lab, I wanted an underground storage room to contain dynamite. So I add a big stack of dynamite onto the map. The map is your visual aid to the world. The encounters are what happens in specific areas.
This may sound odd, but it took me a while to realize that anything can happen in an encounter. There can be a monster, a trap, a pub quiz, an NPC to talk with, something hidden that must be found... the list of potential ideas goes on and on.
So it is easy to add anything. This is where you need to keep in mind your story hook for the whole game. What motivates the players to be on this adventure, and does the encounter help further their ambitions? Everything that happens in an encounter should serve the story.
For a one-shot campaign, a game that can be played in one evening, you need anywhere from 5-15 encounters. I prefer a slightly higher number as you can only sometimes predict what the PCs will do. If you have only 5 encounters, the game will be linear and completed in 1-2 hours. However, if you have closer to 15 encounters, you give the players the option to explore a larger world. They may only explore some encounters during the session, which leaves more fun if the players want to revisit the campaign.
Keeping things fresh - Random Encounters
Sometimes players can get caught up in an encounter and may need a little nudge to move back to the story. For this reason, I also keep a list of Random Encounters the DM can throw at the PCs to give them a little push and refocus on the campaign.
This is the approach I take to world-building. I'd love to hear how you build your worlds.