Righto here we go.
I am using my Linksys Wireless Acces Point /Router as an example.
In my house I have 4 PC's. Mine, my mothers, brothers, and sisters.
First thing you want to do to mess with router settings is access your router.
This is usually done at the address (in internet explorer)
192.168.1.1
The default password for my router is admin - but I changed it to something only I know. I suggest you do too. Consult your manual for the default password.
Now look at all the pretty settings! Now once you consult your manual and find the section where you can forward ports, you will be able to forward to IP's.
Now I don't know about all routers, but port forwarding will NOT work on my Linksys router unless all the PC's I am forwarding to have a static IP address. What this means is that each computer is manually configured with a permanent IP address on the network.. (instead of using DHCP).
Now if you are not using a static IP you will have to. But to keep the structure of this post simple, fill in the port range you want to forward and set it to the IP 192.168.1.xxx (xxx = the 3 digit IP of the computer you are forwarding too.. i.e 192.168.1.101)
Now I have my port range open, for the sake of being simple. forward down from port 2300 up to 5190. This way I can host a lot of directX games on the net without a problem and send files over IM's and receive too.
If you want to forward one port only, set the port range in both fields to that port. e.g. 5190 - 5190.
Now set the ports you want to forward to I.P 101, 102, 103, etc.. whatever you need. Don't worry if your PC is not set on that IP yet.
Now once those changes are applied in the router, we go to our Network Connection settings. This can usually be accessed from the control panel (Network option) or if you have the little icon in your system tray just click to the options through that.
This is how I have mine setup.
Ip address selected as manual
192.168.1.101
DNS - you should set this to whatever DNS your router is using. My router has a page listing that shows my internet IP, DNS, etc.. You can get yours from there as well, or consult your ISP for the DNS if you need to.
Subnet 255.255.255.0
Set your default gatway to your routers IP address.. typically 192.168.1.1
This is all you should need to be able to forward ports. Just make sure the IP on your computer is the same as the IP on your router that you set to forward ports.
Don't give any PC's the same IP.
I hope this helps someone out, and was easy to understand. This may or may not work the same on all routers however.
I hope I didn't miss anything.