To run the install script point your browser to the base url of your website (i.e. http://www.example.com). You will be presented with the "Database Configuration" page.

On Step 3 of INSTALL.txt - RUN THE INSTALL SCRIPT you do not arrive at the Database Configuration page, but are greeted with the following warning(s) instead:

Warning: Table '[database].access' doesn't exist query: SELECT CASE WHEN status=1 THEN 0 ELSE 1 END FROM access WHERE type = 'host' AND LOWER('[ip-address]') LIKE LOWER(mask) ORDER BY status DESC LIMIT 0, 1 in [path]/includes/database.mysql.inc on line 172

Followed by one or more warnings concerning headers that have already been sent:

Warning: session_start() [function.session-start]: Cannot send session cookie - headers already sent by (output started at [path]/includes/database.mysql.inc:172) in [path]/includes/bootstrap.inc on line 811

This happens if you have already edited the $db_url variable in settings.php. The installer will no longer be automatically invoked, because Drupal assumes installation has already been completed.

To remedy, run the installer manually by visiting http://example.com/install.php.