1. Login to the webmail program you prefer. You can get there by going to your-domain.com/webmail. You'll need to login using a username (which is the full email address) and password.
2. Once you have logged into webmail, click on the webmail program of your choice.
3. Copy the url in your address bar. It will look something like:
http://your-domain.com:2095/roundcube/index.php
http://your-domain.com:2095/horde/login.php (if you chose Horde)
http://your-domain.com:2095/3rdparty/squirrelmail/src/webmail.php (if you chose SquirrelMail)
4. Once you have the URL you'll be wanting to use, login to your cPanel.
5. In cPanel, setup a subdomain called something like myemail.your-domain.com (you can do this by clicking on the "subdomains" icon in cPanel). Do NOT use mail.your-domain.com as these particular subdomains are already in use on the server and attempting to use them as subdomains can result in conflicts.
6. After you've setup the subdomain in your cPanel, setup a "redirect" for the subdomain (by clicking on the "subdomains" icon). You will need to setup the redirect so the subdomain goes to the URL of the webmail program of your choice.
That's it!