Its simple, it doesnt matter what setup you are using.. you need 3 things:
The PORT on the OUTSIDE of the router that you want to redirect
The MACHINE on the local network you want to send the traffic to
The PORT on the MACHINE on the local network you want to send the traffic to
So, in the case of bittorrent, ports 6881 to 6889 are the ports on the outside of the router that you want to redirect.
You then need the ip address of the machine you want to redirect them to. You can redirect to a different port on the internal machine, but for claritys sake, its best to leave them with the same number.... say your pc is 192.168.1.1
INTERNET YOUR LOCAL NETWORK
6881 [router] ----------------------------> 6881 [192.168.1.1]
..
..
6889 [router] ----------------------------> 6889 [192.168.1.1]
You would do the same for any other ports you wanted to use... say if you were running a web server on your pc (apache, IIS etc..)
80 [router] ---------------------------------> 80 [192.168.1.1]
So, just find our which services you want to run, their port numbers, and add the appropriate rules. Some routers allow you to do ranges of port numbers, like for bittorrent, above, you would enter 6881-6889, rather than adding each one seperately.