Not being familiar with how to configure my htaccess file, can anyone tell me, simply, how I can allow general access to a directory I have set up myself, which has nothing to do with my Drupal installation, but which is currently giving a '403' error when access is attempted.
This should be already possible - Drupal only is handed control of the URL when Apache discovers that a) the requested file doesn't exist, and b) the requested directory doesn't exist. Since your directory exists, Apache should process it as normal.
I suspect, more than likely, that you're missing an "index.html" or "index.php" in the new directory, and Apache hasn't been set up with a proper DirectoryIndex. Looking in your error_log will tell you the correct error (403 is not immediately helpful).