1. Extract the Modding.zip file to the game directory. (Steam\SteamApps\Common\Spintires).
2. Extract the contents of the Media.zip into the Media folder (Usually you can just Right Click the file and select Extract to Media\).
3. Do the same with the Shaders and TexturesCache zip files (Same as above but for the Shaders\TexturesCache folders).

Now the game wont use the Media.zip, Shaders.zip and TexturesCache.zip files anymore. But you should keep anyway them in case theres an update.

1. Delete the Config.xml file in the User folder.
2. Delete the Media, Shaders and TexturesCache folders (Optional, the game won't use these files anymore).
3. Right click the game on Steam, click Properties, then Local Files and click Verify integrity of game cache.

If the mod you are trying to install asks you to put the files directly in the Media\Shaders\TexturesCache folders then just do that.
If the mod asks you to put the files inside one of the zip files, instead of doing that you should just move the file to the folder with the corresponding name with the zip file you should install.
I.E. if you arew installing a truck tweak that asks you to put the file in Media.zip on the "classes\trucks" folder, you should copy the file directly to Media\classes\trucks

- After making these changes you might lose your save game.
- Multiplayer will work with this tweak, however after you install any mods it will probably stop working.