The plug-in is very easy to set-up, but you have to have some knowledges. Here is how it works in my projects.
The tricky part is to set-up an Apache and dns redirection from the domain1, domain2, etc to the maindomain. The result should be that when you type www.domain1.com you see the content of www.maindomain.com (but keeping the url www.domain1.com in the adress bar)
Once this is done, you just have to configure your different domains with templates, menus, etc in your joomla configuration.
I have really to thank the author of this plug in. I accepted a contract where I had to set-up a multi-site/multi-domain back-end (Only one back-end for several Websites). I did not know that joomla had such a big lack in this domain (and the client wanted to keep joomla, since he's working on it). This plug-in literally saved me and my work, and this for free.
THANK YOU for this masterpiece!