Dungeon Mastery: Campaign world building

1. Map the World

This gives the DM a big picture that makes Dungeon Masters happy - It stabilized your brain so you have a fairly good idea where things are going to go. Knowing how to use MS Word's draw functions is a good idea.

At this point you have a world map.

2. List a few thoughts on what cultures are like.

There is plenty out there...and in your brains. What races are running about?

3. Develop the local area. While PCs dont get to survive long enough to escape, You will likely develop enough to fit a gazetteer - over time.

If you write down a hundred ideas in a list and see where they fall into this Layout:

Dungeon Masters Guide
Introduction.
History as the Immortals Know it.
Geography.
Communities and Settlements.
Society.
Races.
Social Standing.
Religion.
Societies and Organization.
Language.
Currency and Trade.
Governance.
Crime and Punishment.
Relations with Other Nations.
Military.
Monsters.
Artefacts, Treasures and Hidden Places.
Adventures.

and this:

Players Guide
History of region.
Creating Characters.
Special Rules.

And plunder google image search for all the images that would seem to fit into your campaign setting - the real world is an awesome source of ideas.

Don't worry if it doesn't all mesh together and doesn't look like the finished product they turn out of WOTC or Paizo..that will come as it develops.

4. Accumulate Art.

you can plunder inspirational concept art/photos from the internet, employ a starving artist, or best yet...develop your own.

5. Map your local dungeons.

the adventurers need some place to scream and run...again MS word or MS paint with a blank graphpaper jpeg are useful.

6. Flesh it out and write that module.

Module writing. It can hard to make a sandbox into a module. These are usually campaign events whose outcome must determine future events. Evil cult solidifies control over castle ruin? Region in trouble.

7. Short fiction.

This helps build the setting.