1. A method of transfer.
2. A usermap editor, the best out there so far is called 'Forge', it can be found here: http://forums.halomods.com/viewtopic.php?t=78473
3. The HaloMods container tool: http://forums.halomods.com/viewtopic.php?t=78149
Section 1 - Transferring
This section deals with the transferring of files to be modded. This process will require some extra hardware, your options will be described in the following list:
1. XSATA. This is a device that sits in between your hard drive and Xbox 360, and can be used to transfer files from said hard drive to a PC.
2. Xport 360. This is a device made by the same company, but is more flexible in terms of mobility options. It is a USB connected dock for your PC, that can have a hard drive or memory card attached to it. This is the device i recommend for this tutorial.
3. Memory Card Transfer Kit. This device is made specifically for users who do not have a hard drive and use a memory card.
Next you will need one of two transfer software's made for these devices:
1. Xplorer 360 ( found here : http://www.xbox-scene.com/xbox360-tools/Xplorer360.php)
2. Xport 360 ( found here : http://us.codejunkies.com/support/suppo ... ?prodID=32)
Now that you have an appropriate set up for transferring content, you will need to of course get some content to transfer. Start up Halo 3 with the appropriate storage device connected to your Xbox 360. Go into the Game mode called 'Forge', and place objects you would like to change. An example of this would be as follows:
I placed this grenade

And modded it into this clone

Once you have placed all the objects you would like to change, press start and save the map. Once the map has finished saving, turn off the power to your Xbox 360 console and remove your storage device. Connect it to your PC via aforementioned transfer hardware, and open the software. It should look like this:

Hard Drives:
Now, if you are using a hard drive you will want to go to 'Partition 3', then there should be a folder called Content, then one for your profile next, beginning with E000. Next you click the Halo 3 saves folder, which is labeled as '4D5307E6'. Next go inside of its subfolder which should be '00000001'. This will display all of your saved content. Find a file named UserMapxxxxxxxx, this will be the map we saved earlier. Highlight this file, and drag it to your desktop. You are finished transferring.
Memory Cards:
If you are using a memory card, you may have to install additional drivers for it to function correctly. Check you user manual. Now once you have installed the appropriate drivers, the software should display the following two partitions:
IMG
This process is very similar to the hard drive, go to Content, your profiles folder (beginning with E000), then '4D5307E6', and finally '00000001'. This will display all of your saved content. Find a file named UserMapxxxxxxxx, this will be the map we saved earlier. Highlight this file, and drag it to your desktop. You are finished transferring.
Section 2 - Modding
This section deals with modding your transferred file. Open up the program called 'Forge'. Go to the top left of the program, click open, and select your file like so:

Forge should now display a list of tags that are currently spawned in the map.Look for the item you spawned on the map, in my case a frag grenade.

Select the tag, then the editor should appear on the right.
Mod the tag as you desire, in my case I will be changing it into a Spartan Clone. This will be done by going to the the tag swapper in the middle of the window, changing the class and then the tag like so:

Once you are done modding your map, click the save button for each section modded, and close the map & program.
Now that we have modified our map, it needs to have its hash tables and signature fixed. Luckily for us, the HaloMods Container tool does this for us. Open the HaloMods Container tool and open your map like so:

It should have your unique IDs for your profile and Xbox 360 in it, save these IDs and press the resign button.
Now all you have to do is transfer it back to your storage device, do this by going back the folder you transferred from, and dragging it into the window and dropping it. After the transfer is done, hook everything back up to your Xbox 360 and play your creation.
Credits for this tutorial:
Supermodder for the Forge app
Grimdoomer & Staff for the HaloMods Container Tool
If you have any questions, feel free to post them below or contact me on AIM at : xxTheEazyBxx