Skinning Blizzard Default Frames

[Layback_]

For some overhaul UIs like Elv and Tuk, they have created a custom skin (or design(?)) for game menus and some other Blizzard default frames.

I was also expecting to create custom skinned menus, but was not sure of what would be the best practice to do so.

So, if anyone has an experience of creating a customized menus or at least have some knowledge regarding such topic(?), could I get some advice or explanation on how I would achieve this?
(Like process that I should follow: Disable default UI designs, create new design, blah blah blah)

Thank you.