Name your subdomain: xxxxx.site.com (where xxxxx is what you want to call it) If you go with just "m" then it will be m.site.com.
Where it asks for the "document root," simply enter the folder name, in which your Joomla folders reside, which is usually "public_html." That way it points directly to your joomla files, where Mobile Joomla can pick it up, and use it.
Then click "Create."
Now to change mobilejoomla's options to match:
To go Mobile Joomla! Extension configuration, in your back-end.
Click the "Global Settings" tab.
Select "Yes" on "Domain (multisite) support."
Select "redirect to domain" on the four (4) different phones at the bottom of the global settings tab.
Enter your new domain name (eg: m.site.com) for each of the phones. (I'm using one domain for all phones.)
Now go to your plugins, and search for "mobile," and enable all plugins below:
Mobile - Domains
Mobile - Forever
Mobile - Joomla!
Mobile - Simple
Mobile - TeraWURFL
It's a very good idea to put your mobile devices on their own subdomain, because Google, and other search engines love this practice. You will gain a lead in search engines due to this factor. It shows that you care for your site, gives you extra links to your site in their search engines, and you WILL gain pagerank, and domain authority (which is very important.)
Also, it will give you a VERY easy way to customize the template, and layout, because of Firebug, which is an addon for firefox. Firebug allows you to right click: and see the properties of your website, as you see them. So if you, for instance, made your mobile domain "m.site.com," then you will be able to browse to m.site.com on your home computer, and simply right click -> Inspect element, and work on changes easily!
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