The first thing to do would be reading the manual of your router or browsing the support sites of the product.
In most cases the configuration of different routers is similar, so read other posts in the forum, too. Basic steps would be:
1.) Accessing the router on it's IP through a web browser, e.g. 192.186.0.1
2.) Looking for something like Virtual Server, Local Server or Port Forwarding.
3.) Configuration: Normally you have to put in an external port or port range and an internal port or port range. For bittornado you have to forward at least port 6881 (default port) and more (6882, 6883...) if you want to run more than one instance of bittornado running at the same time. So put in the ports, internal and external are the same. The type is TCP, if you have to specify this. Then you have to put in an IP, it's often called local or private IP. That's the IP of the computer that will run bittornado, e.g. 192.168.0.2. You should find that IP on a status page of the router or directly on the pc. In windows ME/98 click Start, Run.. and type "winipcfg", in windows XP/2000 type "cmd /k ipconfig".
4.)Checking the settings: Start downloading a torrent, go to the Details and copy the info_hash, then go to
http://btfaq.com/natcheck.pl and put in that info_hash, if it says "Pass!", you are done.
It's good to disable your firewall as long as you are configuring the router, just to be sure it doesn't block anything.
The default ports of bittornado can be configured in the prefs page.