Develop a core story idea or concept.
Build out the world around that idea - its rules, history, characters.
Create a "Story Bible" detailing this world.
Decide which parts of the world/story go on which platforms (film, game, website, etc.) and why.
Plan how audiences will move between platforms.
Develop the specific content, guiding the audience.
Release the content, guiding the audience.
Interact with the audience, foster community.
Oversee the entire interconnected project.