Is there any easy way to do it?
Assuming you have Windows, if not that would be pertinent information.
I don’t know how to specifically, but I’m guessing it’s the same as non-Steam modding. So you first have to find the install location. By default/on my computer, it’s “Drive:\Program Files (x86)\Steam\steamapps\common\Doom etc.” Remove the (x86) part if you’re running a different version of Windows. Copy the appropriate files over, as described in the readme.