Tony,
Are you talking about simply customising the user login page?
If you are using 4.x (I'm not familiar with 3.x) you can do some useful things with debug. In the admin turn on debug output, debug templates and list of used templates.
Go to the page you want to override (you may have to logout to see the login page!). Scroll down to the very bottom and you'll see which template is being rendered and a button to override it. This will do the necessary work for you in terms of updating the correct override.ini file and creating either a copy or a blank tpl file for you to edit.
