1: download the HPL 2 level editor
2:plan out your map
3:download a good text editing program (I use BBEDIT, i guess you can use notepad or notepad +)
4: spend a good time looking at tutorials (
http://wiki.frictionalgames.com/hpl2/tutorials/start)
5:watch videos to get a better idea some good youtube videos (just type in amnesia tutorials)
6:learn to script
7:test out your script
8:basically refine your skills
and last but not least you would need to spend a lot of time in the amnesia custom stories and modification thread trying to find the answers to your map making problems