I would say the editor is by far one of the easiest one I've ever used but still offers the creative room to make pretty much exactly the custom story of your liking. Might take an hour or so of trial and error but when you get into it it's very rewarding
(Note: scripting on the other hand can be a hassle, but check out the tutrorials and other peoples custom stories to get the idea how it works)
The editors is a seperate download from the Frictional Games site. Just download and stick the files inn your amnesia redist
Kurton: It's like arranging peices from sets (walls and objects). For floors and ceilings you'd make a plane and apply a texture (this can be done for walls also).