Learn
Unreal Script, then download the KF2 SDK from Steam library/tools. After download is complete, look at the files in the SRC folder here C:\Program Files (x86)\Steam\steamapps\common\killingfloor2\Development\Src there is everything you need to understand how things work in the game.
You will then be able to create your custom content.
Pro-tip: if you have absolutely zero knowledge in coding, forget about that idea, first learn unreal script and start with small things.