One way to do this is to start with a simple template - ie atomic. In that template, delete everything between your <body> </body> tags. Copy and paste into that space everything between your <body> </body> tags from your existing template. Delete all info in the template.css file and paste in the info from your existing template.css file. Update the templateDetails.xml file. Delete images in the template/image file and add your images.
There will still be other links, modules, etc that will still need to be updated, but this will give you some idea on where to start.
Another template tutorial is
http://www.virtuosimedia.com/dev/php/ul ... joomla-1.6 This one does include numerous template variations (i.e. color, font, etc) which to me are totally unnecessary when building a 'simple' website. It does have some good info about what a template needs.