The Halo Editing Kit, often abbreviated as HEK, was released by Gearbox Software along with the Halo Custom Edition. It includes, among other things, basic "tags" for bitmaps, vehicles, weapons, bipeds, and other necessary elements of a Halo map.
The kit comes with three programs designed to assist mappers.
The first program is Tool. Tool is both the backbone and the pain of CE. Although it is capable of everything from bitmap creation to the final map-building, its old-style command prompt methods have been rather frustrating for at least a few mappers.
The second program, known as Guerilla and has the icon of what appears to be Che Guevarra, a guerilla leader, edited to look like a gorilla (pun intended), opens tags and edits them. Tags are files that consist of everything in a Halo map: weapons, vehicles, etc. Guerilla can edit them so that mappers can alter what specific item, what it does and how it looks (such as change a vehicle's color). Guerilla tends to crash when it encounters corrupted tags.
The third program is Sapien, continues the primate naming scheme. Sapien opens scenario tags and creates a visual representation of the final map. Mappers can insert vehicles and weapons, place spawn points, and make camera points for cutscenes. Everything that can be positioned in the map physically is done with Sapien. This is widely considered the easiest program of the three, and is comparable to Halo 3's Forge. However, Sapien is slow-loading, and tends to crash often; its debug.txt file is useful when attempting to diagnose such problems.
The Halo Editing Kit also includes a tutorial in the form of a step-by-step walkthrough of the making of the accompanying tutorial map. The tags that come with the kit are almost singularly made for this small map and include select bitmaps from the Campaign and Multiplayer maps. The combination of the tags, programs, and tutorial gives players a glimpse into the unique possibilities of the Halo Custom Edition.
Also, there are some programs that prove to be very useful such as Gmax/3DS Max for 3d modeling, and Photoshop for creating TIF images for creating bitmaps.
Experimentation by Players
Because the HEK is only a basic group of files, CE mappers had to experiment with it and share their discoveries on the workings of the game in order to form a useful knowledge base for editing. Over time, they realized that most of the game's tags were excluded from the HEK, eliminating a huge amount of creative possibilities.
It was at this point that people began tinkering with the HEK programs. Kornman produced a new version of Guerilla called "Kornman00", which unlocked all the grayed-out areas in the program. The new program allows for editing of terrain bitmaps and AIs, as well as many other previously impossible actions. He also created a user-friendly version of tool called "Tool++" and an unlocked version of Sapien called "a_hobo".
The largest breakthrough in regards to tag availability was Steelix B's "HEK+", which allows anyone to enter any original or custom map and extract any tag they want. Along with a few others, Steelix recompiled the entire Halo PC game into its basic parts. CE mappers now are able to do nearly anything they want, including programming AIs into maps, adding usable Pelicans and Longswords, easily creating Campaign-to-Multiplayer conversions or ripping the BSP from it. Also, this program had a feature to "protect" maps, which denied access to tag extraction. However, there was an encryption key, which could unprotect maps as long the key matches the one used to protect the map.
With all of these new tools, a group named "Custom Map Team" (CMT) re-created the campaign with modified skins, Halo 2 weapons, and new scripts. Shortly after the release of their CMT version single player maps, they began work on Single Player v2 (SPv2), using Halo 3 assets and expanded universe ideas such as adding entirely new landscapes, smarter AIs, and even Brutes to the levels. After a few years, they finished their tagset and released SPv2, but it was rushed and was never fully completed. The team disbanded afterwards.
In the July of 2011 they regrouped and began another project, a complete recreation of the campaign - Single Player v3 - this time without half-baked work, and with full integration of Open Sauce. The team is currently developing the campaign, and have so far released .yelo and .map versions of the level Truth and Reconciliation.
Now, many advancements have been made within the program, the maps, and even the mappers themselves. Expert scripters such as Kornmann, Rec0, Bitterbannana, and Tiamat have made various scripts to help boost the interest in the maps.