BuddyPress.org

? what do you not get about the code snippet you pasted, what exactly is the stumbling block?
edit/ oh you mean pasting code into this site? so this example was posted in case useful.

Was the plugin pcwriter mentioned no good?

If you want an even simpler but far from foolproof approach simply use the super global $_SERVER to run a check on the IP and allow the page to run if matching to a set IP or ELSE return a simple html under construction page. Of course this relies on you having a fixed IP