#2 Mar 18, 2005 20:17
That page will point you in the right direction. skins/yourskinname/_main.php is just xhtml with a few tags sprinkled around that make b2evo work.
That page has the general idea of evoskins, and this one is the Templates and Skins category in the manual.
Once you've had a stab, feel free to post any further questions.