Tutorial:
1.) You go into your game directory (e.g. Steam \ Steamapps \ Common \ Farming Simulator 22)
2.) You go to the Shared section and scroll down a bit, so it says Splash_fall, Splash_spring, ...
3.) You copy the backgrounds that you have downloaded and insert them in the shared order
4.) LS22 restart and that's it
Have fun with the backgrounds :)