Instructions
Select "Forge" from the options menu of "Halo: Reach" to open the game editor.
Create a custom map as you would for a regular game.
Select ̶0;Game Type," choose ̶0;Infection̶1; and press ̶0;X.̶1; This will set the game-type to Infection instead of a regular game.
Select ̶0;Advanced̶1; and then ̶0;Infection Haven.̶1; You must have an Infection Haven in order to run an Infection game.
Place the Infection Haven somewhere on your map. Infection-mode will not run and zombies will not spawn unless you have a Haven on the map.
Select ̶0;Spawn Type̶1; and then ̶0;Blue.̶1; Blue spawn points are for zombies; red spawn points are for humans.
Place the blue zombie spawn point somewhere on your map, and then save and exit.