This is done in the router settings itself. You access this through your web browser. The instructions that came with your router should include the default log in details (it varies by brand as to how to get there).
Once you are in go to the Advanced section and look for a section called Port Forwarding, then it's just a matter of filling in the blanks. You'll see the ports needed up in post number 17 of this thread, just need to scroll up a bit. Once everything is in click the button to save the settings.