Well, you should start with following the steps to set up custom stories, and you should install the level editor.
Here are 3 great introductory (did i get that word right?) videos:
http://amnesia.treesoft.dk/tutorials/Cus...Setup1.wmv
http://amnesia.treesoft.dk/tutorials/Cus...Setup2.wmv
http://amnesia.treesoft.dk/tutorials/Cus...Setup3.wmv
Helped me to setup the editor. Way easier than the text version IMO
Also before you start making the mod, train to make maps and to script. Jumping on bigger projects can seem exciting at first but the lack of experience with the editor can take your motivation away. Happened to me a lot :3
Best of luck to you!
''Sick, twisted child... You'll burn for this!''