How to use custom subdomain.domain.com for multisite domain.com/subdirectory?

Okay, so I have done a fresh WordPress installation at http://informables.com which is working fine. I have also managed to enable WordPress Multisite, where I have chosen for ‘Subdirectory’, that means, new sites in the network are going to have http://informables/abc path, which is also fine for me.
Say, I have created following sites in my Network-

• http://informables.com/abc
• http://informables.com/def
• http://informables.com/ghi
• http://informables.com/XYZ

The trouble starts when I want “one” of these sites (say, http://informables.com/XYZ) to be accessed at “XYZ.informables.com” How to achieve that? I have installed the WordPress Multisite Domain Mapping plugin but don’t know how it works.
(Please note that I want the rest of sites (as subdirectories) to work as usual and only want one of those to work as a subdomain.domain.com)

I tried to find a tutorial or some kind of help to achieve that but could not succeed. Basically, the question is about using both subdomain.informables.com and informables.com/subdirectory at the same time without having to choose only one.

I did find some tutorials but those were about using a totally new TLD for network sites (which was of no use for me).

Kindly help me with that.

Note- As it is a new installation, I have nothing to loose. Please suggest anything that it takes to achieve my goal. I just need to make sure I don’t get trouble in in future. Any kind, helpful people there? (I hope there are many… 🙂 )