Your best bet would be to start small. Pick an event, have a look through at all the code and try to figure out what it all means by cross-referencing with the modding manual. Some events even have comments that explain things as well, so that might help. From there, you'd be able to add or remove effects as you see fit.
As for a comprehensive modding manual, I did start writing something long ago...never got around to finishing it. Ho hum.