I'm looking to create my own WHM/cPanel skin and was wondering if there were any good resources or how-to guides available. Thanks!

I'm not aware of any how-to guides, but I've customized 10+ skins. My best advise is to just start modifying the skin to your needs. cPanel 11 is a bit easier to customize IMO, just make sure you backup everything and take it slow.

Once you've done 1 skin it's quite easy to do another.

Agreed, just make a copy of a cPanel theme and proceed with modifying it to your needs. I recommend editing the copy as if you edit the actual theme directly, your customizations may be overwritten by a future cPanel update if there are improvements made to the base skin.

I recommend using X3 as a base because it is cPanel 11 aware. Note that our theme editor for X3 is fairly flexible as-is especially when you start performing CSS customizations. Depending on what you are trying to do, you may not need to customize the code of an entire theme.