Hi Peter,
You do not say which version of Moodle you are using, so can you tell us please then we can help you?
Generally speaking the easiest way is to follow the README.txt in the Clean theme. Alternatively reading about the Clean theme may also help. Also if you look at the top of this Themes Forum page you will see a list of links to tutorials that have been written over the last few years all about Themes and how to make them.
Hope this helps?
Mary