I agree. Guides aren't very detailed about things like these.
You need to have some basic information how skinning in Lotro works. I guides here should cower that part.
Skindefinition.xml is the file that controls the skin.
In short you should know the basic structure of the skindef file:
It should contain <opt> and </opt> tags. these start and end the skin file. everything goes between these two tags.
It should contain SkinName tag. This will present the name that will show up at the in-game menu so you can choose the skin from the list.
Code:
<opt>
<skinname name="my skin"></skinname>
... the modifications (<mapping>, <element>, <panelfile> etc tags) ...
</opt>
You can combine different parts in that middle section. Only difficulty is to figure out what to use. Note that if you use incorrect functions the skin might not load at all (even one incorrect character might break the skin).
Remember if you have dublicate tags in your file the ones that are written first will override the ones that will follow.