Domain Mapping

Hi, Many of us and my clients register our domains at but host our content with other providers or just keep a domain without any hosting.

I have tried changing the Nameserver at Godaddy for my domain to and but do not have access to DNS because I do not have my hosting with GoDaddy.

Can someone give me a SIMPLE step by step guide as to how I can Map my GoDaddy registered to my WPMU client site eg:

I may not have explained myself clearly but I hope this all makes sense to someone!