Don't know why I haven't replied sooner - sorry about that
As far as I know no such document exists - there might be one but I've never seen it. I found that 95% (99%?) of skinning came down to working out what I wanted to see, working out how to implement it (or how it was implemented in other skins if it was an idea I was copying) and then trying to work out where that code had to be placed in order to get the result I was after. A lot of trial and error goes on behind the scenes (well in my case at least!)