Open a web browser like internet explorer or Netscape.
Enter the ip address of your router in the address bar of your browser. In the picture above the address bar has http://www.google.com in it. Just replace all of that with the ip address of your router. By default the ip address should be set to 192.168.1.1.
You should prompted for your password. Enter your password now. By default the password is admin. Click the Ok button to log in to your router.
In the menu on the left hand side of your screen, click the Firewall link.
At the top of your screen click the Special Application tab.
Click the New button to create a new profile.
Use the Choose Application drop down box to select a service to forward. If you do not see the service you wish to forward in this list, select Custom. With other selected your screen should change a bit.
And that's it! You're done!