Here I present a step-by-step guide to building a medieval fantasy world for your D&D, Shadowdark, Pathfinder, Old-School Essentials, or Low Fantasy Gaming campaign.

1. Decide what sort of adventurers and adventures everyone is interested in.
2. Decide on the concept and central tension of the world.
3. Decide what major creature populations you want in the world.
4. Name the places those populations will live in (homelands).
5. Create a history of the realm.
6. Decide how that history generated ruins and dungeons.
7. Decide on the social structure that came out of that history.
8. Come up with some other major geographical features.
9. Create a map of your realm or continent with 18-mile hexes.
10. Label the map with your homelands and points of interest.
11. Label the map with the major cities and perils.
12. Decide on two or three factions and make them matter.
13. Write up a brief description of the realm.
14. Create a map of each starting area with 1-mile hexes.
15. Label each 1-mile hex with terrain and points of interest.
16. Populate the starting area maps with towns and perils.
17. Populate your towns and cities with townsfolk.
18. Write up a brief description of each starting area.
19. Describe the major cities, especially around the starting area.
20. Invent a rival NPC company or two.
21. Add some specific NPCs to your starting city.