1) Open a web browser window and type your Default Gateway IP number in the address bar. It should be 192.168.1.1 in most cases.(Depending on your router, it could be a number of addresses.) Check this site for a list of default router passwords.
2) To login leave username blank and type admin for the password (unless you have changed it before which is always advisable)
3) Find the section that says Port Range Forwarding or something like that.
4) Add in the port range 6881-6889 for Torrent files in the window.
5) Make sure you check both protocol boxes TCP and UDP as well as the Enable setting.(important)
6) Have the forward address set as your internal IP address 192.168.x.xx , To find yours Go to Start -> Run and type in cmd. This will open a shell, type the command " IPCONFIG " and look for your IP Address.
7) Confirm and submit all the changes and that will probably work, if not , and you have several computers on the internal network, you can always turn on Port Triggering (in Advanced) telling it to forward the ports you list above with the trigger port set to 6881 and the protocol to TCP. This way you don't need to know the IP leased to the specific machine.
If i left anything out, tell me, and I'll change it. The ports might be different, so you may have to check the specific torrent client. Can usually be found in the preferences tab of the client by " checking " the connection to see if others can connect to your machine.