Just stick it on right away will not work. To do this, you need to unlock them for 50,000.
Here's the way:
Go to the car, click LMB (customize) and scroll to 10 of the "Livery" section to buy.

You can download ready-made ones on the Internet. If you have done so, proceed to the next point.

• Go to steam, click on the game settings, properties, files, show installed files.
  
• In the window that opens, select "_LIVERY_TEMPLATES"
  
• Click on the folder with the name of the car you need. (at the time of creation, only SX is in the game)
  juice

And so now you have a template. Upload it to any photo editor you like (photoshop, pixlr, etc.) and you can glue whatever you want. Please note that glass can also be painted and vinyl glued. Once you're done, save it to any folder convenient for you, as a png with a transparent background.
And so, you have a livery from the Internet or made in person. The main criterion is the format the game needs , namely: 4096x4096 or 2048x2048. If he is different, then the game simply will not accept him.
Personally, I came across this and, unexpectedly for me, Paint 3D helped me with it.

• Opening the finished livery
  
• Go to the "Canvas" section
  
• And set the format we need there
  
• Save as png.

Now everything is ready. Go to the car, click lmb (customize) and scroll through up to 10 of the "Livery" section. Lmb. In the window, select the livery.

Thanks for attention. I hope I helped you and saved your time. If so, rate my work and if you want to reward me with a community award, I will be grateful.
If you are interested, rate my other work:
https://steamproxy.com/sharedfiles/filedetails/?id=3441337063
Have a nice day!