Halo Custom Edition maps go in a folder called custom_maps, beside maps. Don't put them in maps itself.
custom_maps also needs three shared files from your own Halo Custom Edition install:
bitmaps.mapsounds.maploc.map
The Gearbox PC maps (Ice Fields, Death Island, Danger Canyon, Infinity, Timberland and Gephyrophobia) aren't included. Copy them from your Custom Edition install's maps folder into custom_maps if you want them.
Once the files are in place, the maps show up under CUSTOM SINGLEPLAYER and CUSTOM MULTIPLAYER in the map lists. A custom singleplayer map plays as a campaign level, alone or in online co-op.
Maps that need OpenSauce won't work.
Automatic downloads
When you join a game on a map you don't have, the game downloads it from the other players, along with any of bitmaps.map, sounds.map and loc.map you're missing. This works on a LAN and online, as long as the host has map sharing on (it is by default).