Instructions
Determine the rule set you want to use for your custom campaign, then obtain the appropriate rulebooks.
Design your campaign world and develop a theme. The details of a magic-filled world will be quite different from one with no magic at all. The more detail you create for your world, the more dynamic it will feel to the players.
Create a campaign story. This story can be open ended, allowing players the ability to highly affect the direction and flow of the story, or it can force players down a predetermined path. The choice largely depends on the preferences of those involved.
Find players interested in playing through the campaign. If you do not know anyone personally who would like to participate, checking your local hobby store can often yield good results.
Meet with your committed players and play.