Instructions
Load Halo: Reach on your Xbox 360.
Sign into your account (the one you used to create the custom map), when prompted.
Choose the storage device that your map is saved to from the next prompt. This is likely your hard drive.
Select "Main Menu" from the title screen.
Select "Custom Game."
Highlight "Map" then press "A."
Select "Local Files." This allows you to view the maps saved to your profile.
Select "Forge World" from the maps list.
Choose the name of your map variant from the drop-down list of maps existing under "Forge World." The map will take a moment to load, as indicated by the on-screen progress bar.
Select "Start Game" to begin playing your custom map.