You need to have a USB drive formatted to transfer files to/from your 360. Well, you don't *need*, but it's the easiest method.

Using the USB drive you transfer the game variants you want to modify from your harddrive (or just use the USB stick as your content device, flat out skip a step).

Then connect the USB drive to your computer and launch Horizon. Note: you don't *need* to have the drive connected before launching Horizon, it should autodetect Xbox formatted devices.

Using the device explorer tab, navigate the "Games" virtual folder, and then find the Halo 4 folder. From there, you should have a list of the Halo4 content files (could contain map/game variants, films, etc).

Note: you will need an xboxmb.com account with a Diamond subscription to use the Game Variant editor at the time of this writing. Other game save and profile editors are exclusive to Diamond as well. Eg, there appears to be a lot of excitement for the Bioshock Infinity support that was added in 2.6 as well. Eventually, some more of my research will be turned into more user-friendly editors for other Halo games (including Halo Wars!), so I recommend at least a month subscription for 4.99$.

From that content file list, open (double click should work) the variant you wish to edit. It should bring up a window that looks like the third image from my OP.

Edit whatever values you want (more will be exposed in the coming days), then hit the Save tab. This should save and rehash the content file.

If you're editing straight from the USB drive, you're set to eject it, plug it into the Xbox, and take the changes for a spin in-game.