I have asked this very question a gazillion times and I can tell you right now...you won't find a tutorial. Even taking a class, and paying for it, doesn't help much with the answer. What I believe the answer is (and I could be wrong) is to first have a strong background in HTML and CSS styles. Then, pick a default template that comes 'close' with respect to region layout. Next thing is to go to the template definition and do a COPY of literally EVERYTHING...give them all new names. Then you get the arduous task of editing each of the template subpieces until you have something satisfying.
Personally, I hope I'm wrong about this. It would be nice if there were a Template Building wizard in APEX.