SlowMotion with the xBox Controller

Hey guys!

So I want to setup the slowmotion options for the xbox controller,but just dont know how. It really seems to be a change for lazy guys, but to reach out for the keyboard is kinda annoying and you also miss the crash when your car is driving about 200 km/h.
I looked up in the forum and also found one setup, but it wasn't as i wanted it to be like for myself...Furthermore I don't even know how to activate that setting, as the game still used the normal xbox configuration, as i tried.

Can anybody explain me how to add the ''0.5x slowmo'' onto the LB button and the ''back to realtime'' onto the start button and how the game uses this config rather than the ''normal'' one?

I dont know a freaking thing about coding and stuff, so a very simple explanation (so that even total retards can get it) would be really appreciated

Thanks in advance!