To begin, you will need a PHP-enabled webserver like apache or apache2 with the php4 module.

SquirrelMail runs on top of an IMAP-server. Having an IMAP-server installed and working is a pre-condition.

Install SquirrelMail as per the installation instructions.

Now to make the install web-accessible. With the configuration files you'll find an Apache configuration snippet you can edit and add to your Apache config. It's wise to add it to the configuration of Apache-SSL as well, so your users can access their mailbox over a secure connection. After that, reload Apache.

Now go to https://webmail.example.com/src/configtest.php (if you configured SquirrelMail at the webmail.example.com virtual host). If this configuration test doesn't display any errors, proceed to https://webmail.example.com. You will be presented with a login screen. Enter your username and password (the same as your system login). If all is correct, you should see your mailbox.

If it does not work, check whether your browser has cookies enabled. Also verify whether your IMAP server is working correctly (in its logs, and/or with a regular IMAP mail client). You can also check your webserver logs for any error messages.