I don't think there are any good tutorials. Nor does Turbine provide any documentation. The only way to figure out the correspondence between the <Mapping>s and the <Element>s is by trial and error. And when you do figure it all out, it's a very tedious process to actually edit a skin, because every time you change the SkinDefinition.xml file, you have to quit and restart the game before you can see the results.
Someone made a LOTRO Skin Editor
several years ago, but I think it's severely out-of-date.
Maybe Adra has some tips for how to make the process tolerable, but from what I know I wouldn't recommend it to any but the most patient people.