ALWAYS MAKE SURE YOU MAKE BACKUPS!
To avoid having to reinstall your game!
I also want to point out this way will work if your game is installed onto your system. This will not work via disc, unless you only mean to change the DLC characters.
On the Wii U SD-card:
(You can download these files via the Homebrew Appstore)
On your PC:
A mod you want to use.
Good, You have all the required files!
Let's get your Wii U ready.
First, open the Homebrew Launcher (Either via loadiine.ohv, or via a title where you installed it on)
In there, open Mocha CFW, If text pops up, just press A.
It should return you to the Wii U menu.
Then, open Homebrew once more and open FTPiiU_Everywhere.
It should display an IP adress on your TV / Gamepad screen.
Great, your Wii U is ready for use!
Now comes the PC part.
Start FileZilla and enter at HOST the IP desplayed on the TV / Gamepad.
Your PC should be connected to the Wii U now, and it should look something like this:
From here, you can manage the SD card, Internal memory, External memory and the sys_nand.
If you installed the game onto the System's harddrive you have to enter; /storage_mlc/ If you have it on an external drive, use /storage_usb/
From there open /usr/title/
In here you should see multiple folders.
00050000, 0005000C and 0005000E.
If you would like to change the standart data of the game you should enter 00050000
If you want to change the DLC of the game, You should enter 0005000E
In here you see a lot of folders, These are your installed games.
In order to find the game you are looking for I recommend this site.
Here you can look at the title ID's for games, for example; Mario kart 8 EUR has a title ID of 0005000E-1010ED00
It exists out of two parts.
0005000E for the DLC folder.
And 1010ED00 for the games folder name.
So you should look for the folder 1010ED00 if you are looking for mario kart 8 on a EUR system.
In here are three folders.
Code, Content, and meta.
We want to have the "Content" folder.
(If you want to change the Splash Screen of the game when its starting up, you should look in the meta folder at 0005000E. They are called bootDrcTex.tga(Gamepad) and bootTvTex.tga(Tv))
*In the content folder I recommend making a backup of the files you are going to replace.
In this content folder you can just drag and drop the mods onto the right locations.
The Sonic mod is easy just to drag into
But character reskins have to go into /content/driver
When you want to replace music you should check out /content/audio/stream
And for replacing wings /content/kart/wing
When you're done here, press the home button, turn off the console. Then back on, check the game and bam. You should've injected your mods right into the game.
Thats it. not that difficult right.
Please, keep in mind to not just mess around in your Wii U's data files.
Sorry if the explanation was not that good, I tried my best to be as clear as possible. I do have dyslexia but I hope you guys got it anyway.
Yes, this is WiFi safe, custom characters/songs are client-side.
And yes, Injecting files/changing boot images also works on other games.
If you have any questions, feel free to ask them.
Also feel free to correct me on anything, and you are welcome to make any translations.