Could you tell me how to use these arguments?

Sure thing! Right click on any game in Steam and click Properties. Then in the General tab, you'll see a Launch Options box where you can paste these arguments in.

What most people get wrong when first trying to use it is not knowing how to correctly specify environment variables vs launch options that get passed to the game executable. If you just want to pass arguments to the game, just paste them into the box. So for example with Cyberpunk, you can just paste in

--launcher-skip

and Steam will launch the game as if you were running

Cyberpunk2077.exe --launcher-skip

However, if you want to specify environment variables as well, you'll need the %command% placeholder. So, in order to enable raytracing and bypass the driver check for ray reconstruction in Cyberpunk, I paste these launch arguments into the settings:

DXVK_NVAPI_DRIVER_VERSION=53799 VKD3D_CONFIG=dxr11 %command% --launcher-skip

which is like running

DXVK_NVAPI_DRIVER_VERSION=53799 VKD3D_CONFIG=dxr11 Cyberpunk2077.exe --launcher-skip

%command% is just a placeholder for the game's executable path.

Hope that clears things up with regards to the launch options.

As far as knowing which environment variables to use, that's on a game-by-game basis, but the two most common ones that I use for Nvidia GPUs are PROTON_ENABLE_NVAPI=1 which enables DLSS in games that are not on Proton's NVAPI whitelist, and VKD3D_CONFIG=dxr11 which enables raytracing. I almost never bother with any other environment variables unless there's special game issues I need to workaround (like Cyberpunk's driver version check), in which case I check ProtonDB or the game's issue tracker on the Proton GitHub page.