Pretty simple once you’ve done it once, a pain if you haven’t. Start by enabling the apache rewrite mod:
a2enmod rewrite
Next, edit your sites enabled config ( /etc/apache2/sites-enabled/000-default ) to allow apache to except overrides from .htaccess files. To do this, edit the 000-default and change the AllowOverride None to AllowOverride All.
In Drupal run the clean URL test ( Admin -> Site Configuration -> Clean URLs ). Then enable radio button should be available now, click it and save your configuration.
