start with a simple template - 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.
Then delete all info in the template.css file and paste it in the information 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, this tutorial will help you