How do I setup domain mapping with domains names hosted on the same server as the WP multisite?

I own a number of domain names which are all hosted on the same server as my Wordpress multisite setup. The IP address for all the domain names is the same.

I want to be able to set up domain mapping so that my clients can use their own domain rather than sitename.com/site1 sitename.com/site2

I can't seem to get it to work despite my googling!

Thanks