Here are some links to get you started:
DirectAdmin Skins Tutorial, The Basics:
https://www.directadmin.com/skin_basics.php
DirectAdmin Skins Tutorial, Extended guide:
https://www.directadmin.com/skins.php
I want to customize the appearance of DA and what it can do:
https://help.directadmin.com/item.php?id=294
How to customize one portion of a DA skin, without needing to create your own skin:
https://help.directadmin.com/item.php?id=295
However I think the links above is most suited when looking at the Enhanced skin and when you want to code it with same type of template files as Enhanced (the new Evolution skin I think uses Vue/json, and I think coding it like that needs some different and new tutorials).