UABEA, used to put your livery into the game. You can download this on GitHub by googling UABEA or by clicking https://nightly.link/nesrak1/UABEA/workflows/dotnet-desktop/master/uabea-windows.zip
Any image editing software like Paint.net (Free) or Photoshop (Paid)
A car template:
RX has not yet got a public template.
Supercup: https://i.imgur.com/p0l2ven.png
RL300: https://i.imgur.com/9bd8mcx.png

Now open up UABEA and press ‘file’ and ‘open’. Then head to the game directory and RaceLeague_Data folder. Here select the file ‘resources.assets’.
Now a window should pop up. Click ‘View’ and ‘Search by name’. Now you’ll have to search the name of the livery you want to change.
These liveries are named as following (At the time of writing this)
For the RL300:
- RL300_skin1 | RL300_skin2 | RL300_skin3
- RL300_skin1_mask
For the Supercup:
- skin_bluecow_blue | skin_bluecow | skin_racetire | skin_racetire_green
For the RX:
- skin_white | skin_green | skin1 | skin2

Be aware, some skins need you to change 2 files with the same name in order to work properly. These are skin_racetire_green and skin_green.

Now that you have found your file, press ‘Plugins’ on the right and then ‘Export texture’.
You can now edit this file using the templates above.

After you’re finished with editing the file, you can head back to UABEA and on the skin file, press ‘Plugins’ and ‘Edit texture’. At the bottom, click ‘Load’ and when you’ve selected your custom file, click save.
Now you’re back again in the Assets info window. Click ‘file’ on the top and press ‘save’.
After you have saved, go to the game directory again, and replace the old resources.assets with the one you’ve just saved.

And that's it, your livery in now in the game. Do know that it will only be visible to you, or anyone who has the same custom .assets file.