Halo CE setup and installing mods
So you've seen some mods and are interesting in Halo: Custom Edition. Unforently, since this game is a years old it is a slight setup. Here's how.
First thing you need is Halo: Combat Evolved for computer. You can grab a copy over at amazon.
You will only need the CD key to play Custom Edition. Once you got the CD key in hand, go over to Halmaps and download Halo: Custom Edition.
Once the download is finished, install using your Halo PC key. Now go grab the Custom Edition 1.09 Patch. There are many ways to update the game (load game up with CD, go to the install and click haloupdate.exe), but
to keep it simple, you can just get an installer over at Halomaps.
This next part is optional, but if you plan on making your own mods for Custom Edition, be sure to grab the Halo: Editing Kit.
Now that you've got the game running, there are a few optional add-ons that anyone should grab. The first is the Universal UI.
This will let you play and run custom modded single-player maps without a hassle of opening the game in Developer mode (see below). The Universal UI supports most popular custom single-player maps, such
as Project Lumoria, CMT SPv3, Takedown, and many more projects. Although it will not automatically update, so if you want to play a custom single-player map not listed in the Universal UI, you must load developer mode. See this tutorial for more.
But you shouldn't have any problems.
To install the Universal UI, or any other Custom Edition map files, find your Halo: Custom Edition install. You can usually find it under "Program Files/Microsoft Games/Halo: Custom Edition". 
Once you find your install, take the ".map" file included in your download and drop it into the "maps" folder in your Custom Edition folder. The mod is now in your game. For the Universal UI, it 
will ask you to over-write UI.map. Say yes, since this is an update to the original UI file.
The last optional addition to cover is Open Sauce. Recently, modders have been exploiting the game engine and pushing it past its limits. Mods that contain a lot of files within them (Takedown: Project Lawson) or
mods that contain a huge graphical and ability update (SPv3) require open sauce to play. You can download the latest version of Open Sauce from Google Code. 
Once downloaded, just install with the installer. This will give you open sauce features. A project that requires Open Sauce will be a ".yelo" file instead of a ".map" file. Though they are to be installed into the
same place. If you have a map and yelo file named the same thing in the maps folder, the game will load the map file by default.
Note: The Universal UI does not come with the maps listed in it. The Halo: Custom Edition game only comes with the default multiplayer maps. With OS 3.1, maps will automatically download for you if you try to play on a multiplayer server which you do not have the map for. To
find singleplayer and multiplayer maps to download for Custom Edition, check out Halomaps.