Okay first things first You have to download the HPL Editor Suite Here
http://wiki.frictionalgames.com/hpl2/tools/start
Download for what you have (Windows, Mac, Linux)
Next your gonna want to either extract it (if it's not extracted already) and put the files in your Amnesia redist Folder (MyComputer-C-ProgramFiles-AmnesiatheDarkDescent-Redist) Or (MyComputer-C-ProgramFiles-Steam-SteamApps-YourUserName-Amnesia-Redist)
Next your going to want to open the HPL Level Editor. (Makes your Level)
Theres Tutorials On youtube about the basic's and such about the level editor
Say One your done making your level and you want to test it you have to do a few things (gonna link video)
http://www.youtube.com/watch?v=7bUyTu_bnSA&feature
you Do the things in the video and your good to go.
For All the Script Functions there here
http://wiki.frictionalgames.com/hpl2/amn..._functions
and I Suggest Downloading Notepad++ for scripting. It helps alot.
Hopefully that helps you
-Grey Fox