The gui is a PHP application, by default it's installed to /var/clapf/www, and you must map a virtualhost to the documentroot of the GUI, eg. antispamgui.yourdomain.com.

The first step is to create config-site.php, which stores local site specific settings. An example config-site.php:

<?php

$config['DIR_BASE'] = '/var/clapf/www/';

$config['BRANDING_TEXT'] = 'SPAM FREE INBOX';
$config['BRANDING_URL'] = 'http://clapf.org/';
$config['SITE_NAME'] = 'antispamgui.yourdomain.com';
$config['SITE_URL'] = 'http://antispamgui.yourdomain.com/';

$config['SMTP_DOMAIN'] = 'antispamgui.yourdomain.com';
$config['SMTP_FROMADDR'] = 'no-reply@antispamgui.yourdomain.com';
$config['HAM_TRAIN_ADDRESS'] = 'ham@yourdomain.com';
$config['SPAM_TRAIN_ADDRESS'] = 'spam@yourdomain.com';

?>

In case of apache 2.x, make sure rewriting rules are allowed, see .htaccess. For nginx you may find an example config in contrib/webserver/clapf-nginx.conf

The default account is the built-in administrator:

 admin@local:clapfrocks

Finally visit http://antispamgui.yourdomain.com, and make sure to change the admin@local password, then create or import users. Start with adding the valid domain names.