But it's a form of modding that covers a LOT of ground. So rather than a simple tutorial, it really deserves a whole wiki on the topic.
The wiki format also makes it easier to get help from multiple contributing authors, since it is such a big area to cover.
The site page menu in the left-hand panel is ordered the same way as the directory structure of a typical custom class mod. So just browse these pages to learn how the contents of these directories are affected when you make a custom class.
To keep the quality of the info on this site as high as possible, it's not a fully open wiki. Anyone can read through it, but you'll need to join the site to be able to edit anything.
Unfortunately, this setting also stops non-member visitors from commenting too. So until we find a solution for that...