Ok I finally got around to developing a tutorial for this environment mixing. I'll detail each step that it takes, its pretty easy once you know the steps.
First though I must give most of the credit to Skubidou for providing guidance as to how to do this, and Mixo as the original author of one of these environment mixes, although mainly unknown.

I tried to create this tutorial so that someone who hasn't mixed in the hex editor before could understand it, but it really helps if you have some prior experience.

The mix consists of taking your desired environment and adding the surrounding background of your choice.
So for example in The_Killer's map it was a stadium track, but with Coast mixed around to the back.

Then I guess, onto the tutorial!
The process:

First:
You must choose your environment that you wish to build the track in. So lets say in this case I want to build a snow map with the coast background. I would start out by selecting coast as the environment in the ingame editor. Once the editor opens up, place a single panel in the editor. Paint the panel, using the a false URL, such as:
http://False.URL.com/Text_To_Delete_To_G..._Bytes.dds
Note: I always use the same URL, the one mentioned above, as that way it decreases the amount of variety and makes copying the mix a whole lot easier.
then save it and close the game. The reasoning for this panel will be explained further on, but for know it will suffice to say that it is vital to the mix.

All you should have in the map is a single panel as such:



The next step:
You have to open up the map you saved in the previous step with ChallengeEdit, then resave it. The purpose for this is to decompress the map so you can properly see the hex. It makes your life extremely easier. Following that, open up this uncompressed file in your hex editor of choice. Don't have a hex editor? Here is a link to the free one I use: [url="http://www.chmaas.handshake.de/delphi/freeware/xvi32/xvi32.htm#download"]XV132 free hex editor.[/url]

Now comes the actual editing part.
With the file open in your hex editor, scroll to the bottom of the file and locate the section 02 00 00 40.
For reference the section you are looking for is highlighted in orange in the following image:



It directly follows the word Day, so if you have trouble finding the numbers, look for the word.
The section you are looking for is the one after Thumbnail/Comments (so first search for </Comments> then for Day).

Now that you found that point, you want to insert the code for your background you want to add directly after the 40 or if you are looking at the words its an @ symbol(at least in my hex editor).

Codes to insert for each environment are as follows:

07 00 00 00 53 74 61 64 69 75 6D - Stadium
05 00 00 00 43 6F 61 73 74 - Coast
05 00 00 00 52 61 6C 6C 79 - Rally
06 00 00 00 49 73 6C 61 6E 64 - Island
06 00 00 00 41 6C 70 69 6E 65 - Alpine (Otherwise known as Snow, Nadeo requires it to be Alpine in the GBX.)
05 00 00 00 53 70 65 65 64 - Speed (Otherwise known as Desert, Nadeo requires it to be Speed in the GBX.)
03 00 00 00 42 61 79 - Bay

Notice that the first bit changes according to the size of the word. So once you have one mix, you can easily change to another *if* they are the same amount of letters. So for our example, once we finish mixing the coast around the snow, all we have to change is the word coast to another to get another mix easily. We could replace coast with Speed for example for a quick change and not even have to do the extra work.

So your final thing after this should look like:



Notice how around the word Coast there is 4 blanks in front of it? There has to be four. The first bit of the four shows how long the word is. So for our example (coast) the four blanks will be 05 00 00 00. There also must be three blanks (00 00 00) after the end of the word, followed by @ (40).

Next step:

Ok, now that we have added a string definition to the file, we need to tell the file that there has been one added, by changing the definition of the next one up by one. So locate the section 80 00 00 05:



and change it to: 80 00 00 06.




Update: Skubidou provided some nice input on this section:

Skubidou on TMC Wrote:Perhaps it is dangerous to search for 80 00 00 05, since 80 is Variation2 of the bloc (so could be another value with another panel in another environment). Better to search for the 05 00 00 40.

Just make sure if you are searching for 05 00 00 40 as Skubidou suggests, to change the value to 06 00 00 40.


Due to the part we just went through, it is very much to your benefit to only use this mix on base maps, with nothing but the panel, otherwise you have to go through and change all the definitions.....have fun with that.


So.....
Now we have added a more to this file to achieve or desired effect. BUT! The GBX has certain checks in the GBX that pretty much see whether or not the file size is the same. This is where the panel with the fake URL comes into play. To make sure it is the same size, you need to delete characters from the URL to match the changes you added earlier. So for each environment delete the following characters from your URL, in the words of Skubidou:

Skubidou Wrote:(where you want. The URL was false start, the end is also false URL ;-)

11 - Stadium
9 - Coast
9 - Rally
10 - Island
10 - Alpine(Snow)
9 - Speed(Desert)
7 - Bay

Here is an image that shows all the changes that should have taken place:



One last step!
Since you changed the length of the URL, you need to modify the Dword of the URL.
If you used this fake URL: http://False.URL.com/Text_To_Delete_To_G..._Bytes.dds
Then the original Dword will be 3E.

So if you are using the fake URL from this tutorial the final Dwords will be:
33 - Stadium
35 - Coast
35 - Rally
34 - Island
34 - Alpine(Snow)
35 - Speed(Desert)
37 - Bay




Since we deleted 9 for the word coast ( or however many for the respective environment you chose), we need to decrease the Dword by 9, so 3E becomes 35. And thats the final change!

In this step you see the benefit of using the same URL everytime. If you start with the same the DWord will always be the same so reducing it will be easy and the same for each time you do that environment.

Now, if all these fairly simple changes have been done, then you should open the track up, and see Snow environment with a Coast background!

So some final notes...
As you go through each environment you will notice they are on different levels, and some are different sizes. So I have created a list of viable, good mixes:

Desert and Snow either way ( they are the same level)( Probably the best out of all)
Stadium With Coast background (Not great, but works, obviously)
Stadium with rally background (there is a slight break in the edge)
.....
The rest have too many inconsistencies and black gaps to be good for building a good map.
(I can already see the amount of crap maps this is going to start.....for all our sakes, please, please, think is your track just showing off the mix or does it actually have merit??)

Another note: when you create a bad mix, the game will crash, and when you reload the game the file will magically disappear from the directory. Its not really gone, it was moved to the CorruptedFiles folder in the main directory.

For the more experienced people:
Lastly something I haven't figured out yet, I haven't been able to mix a Island map and add a background to it yet. Why? I'm not sure, I've followed everything perfectly yet still nothing. The weird thing is, you know where you look for 02 00 00 40? Island is different. It has 20 02 00 00 40. I'm not sure why the extra 20 is there. I've tried changing it, removing the 02, removing a 00, but I still haven't figured it out. If anyone gets it I'd gladly appreciate learning my mistake . Maybe some environments don't mix, I don't know.

So that's the story folks. I know this is a controversial mix, I'm not sure it should really be spread because it alters TM in such a weird way, its almost like blasphemy . So I leave you with the pack that Skubidou gave me to test out the mix. Once you get the first mix and learn the procedure the rest are easy to apply.

Test mix maps to learn the mix.
I couldn't add it to my original tutorial, but here are the links to the bases I have made:

Bases List:
Stadium With Coast Background
Stadium With Rally Background
Stadium With Desert Background
Stadium With Island Background
Stadium With Snow Background
Snow With Rally Background
Snow With Desert Background
Snow With Island Background

Have a base that's not here? Post the link and I'll add it to the collection. I'll also add others I make.


Some Bases Already Mixed.

Any questions? Feel free to ask. I'm sure there are plenty of experts willing to give their opinion.
And for you hex experts out there, see anything I got wrong in this tutorial?

So once again thanks Skubidou for the guidance on this mix.

~MExUnited