Any decent graphics/art modding tutorials?

I'm eager to create custom building/unit graphics to swap into the game, however the tutorials I come across just lose me when it comes to locating the existing art files. (At the moment I'm just accessing files at random and hoping I come across what I'm after)

Are there any decent guides that take me through the process?

I appreciate any responses even if it's just links to random old threads that go nowhere :)