We're assuming your placing the backend folder in /var/opt/backend and the website will be served from /var/www/backend-site.

• Download or branch the code from Launchpad.
• The public folder should be moved into the Web server's document root, eg. mv backend/public /var/www/backend-site.
• The backend folder should go outside the Web server's document root, eg. mv backend /var/opt/backend.
• Edit the index.php file in /var/www/backend-site, and set the appropriate folder constants:

  define('BACKENDFOLDER', '/var/opt/backend');
  define('APPFOLDER', BACKENDFOLDER . '/webapp');
  define('SITEFOLDER', '/var/www/backend-site');
  

• Open up the URL that points to the public folder of backend.
• Click on Install Application Under the Go menu.
• Sign up a user using the standard signup form.
• You should receive an email with a link to confirm the user. This account is the super user account.

If you follow these steps, you should end up with a working backend installation.

Any customizations to the code and templates that is site specific, you can put in /var/opt/backend/webapp, and any changes to the stylesheets or JavaScript will happen in /var/www/backend-site.