You should also take a look at the wiki:
http://wiki.frictionalgames.com/hpl2/amnesia/start
Anything from HPL2 onwards will teach you how to set up a custom story, try a few events such as making a monster appear or making a door blow open.
A noteworthy page on the wiki while you're developing is the scripts themselves. Once you're familiarised with how Amnesia's scripting works, you should also take a look at the various functions
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You can find them here:
http://wiki.frictionalgames.com/hpl2/amn..._functions