Multi Site Domain Mapping Setup

Hi Guys,
I'm really struggling to get the multi site domain mapping setup.
I can't seem to find any detailed instructions so if someone can link to some I would be very grateful.
Basically what I need to do is add a domain to a site.
I am not sure if I need to just use the plugin or the plugin or both. to
I don't want other sites to see or have access to this site.