Theming: Is this a valid way to go about it
You certainly could do it that way, however, that would require you to know all of the php necessary to make a fully functional Drupal page.tpl.  What I would recommend, and how I started themeing (outside of reading a bunch!) was taking a page.tpl.php from a theme similar to what I was looking for and modifying it until I was happy with it.  After doing this several times and experimenting a lot (and reading!) I became quite familiar with the templating system used in Drupal and the php code needed in the most common template files.