Simply install/modify the template the way you want it and then specify the template via a URL parameter such as:
Normal Site: http://www.sudobash.net (Note – I’m no longer on Joomla so these links won’t work – you’ll need to do it on your own joomla site(s) )
New Template: http://www.sudobash.net/?template=ja_purity
Then check it out from there. You don’t need to change your template to the “live” one until you are sure its good. Then just change it via the “default” button in the Joomla Templates section like normal.
You may also find it useful to show the templates module locations, this can be done by adding ?tp=1, or if you are doing both it would be something like http://www.sudobash.net/?template=ja_purity&tp=1