First, know what your computer's IP address. This should be the computer that has the P2P software installed. (How to do this? Go here http://kb.iu.edu/data/ahse.html
Then, open your Internet Explorer and type in on the address bar http://192.168.1.1
and this should prompt you to login. Just type in admin
for both the username and password.
Go to Advanced Setup > NAT > Virtual Servers
Click on Add then Make New One and use the name of your P2P software for the Server Name. Then your computer's IP address for the Server IP (which you have obtained when you did the first instruction).
Then for the rest of info required such as External & Internal Port Start and End and the Protocol, you may refer to your P2P software port configuration.
If you're not sure what it is, check the support website of your P2P software.
Hope this will help. Cheers x