Far more than I can type.
More involved than the physical WAN and LAN ports.
Basic concept is that you want certain network traffic to go to a specific computer and more precisely to a specific port on that computer.
For the most part a port associated with some function, application, or game running on one of the host network computers.
What happens is that the router receives traffic for some specific port.
The router looks at its configuration information and finds that traffic for a specific port is to be forwarded to a computer using one of the allowed IP addresses. There are two IP address types: DHCP IP (by range) or a Static IP outside of the allowed DHCP IP address range.
For port forwarding the destination IP address must be a static IP (assigned to one computer) or the traffic would likely be sent to the wrong computer.
So the process is to configure the router accordingly. The destination computer must have a static IP address - that means the IP address always remains the same on the host network. And that static IP address should be reserved via the computer's MAC.
Here is a tutorial to help out:
http://www.steves-internet-guide.com/understanding-port-forwarding/
You can easily search for additional tutorials or videos. For example:
(Short)
https://www.youtube.com/watch?v=-K6jMYBfuIY
or
LONG
https://www.youtube.com/watch?v=dmQnJRXbzEk
Once you get a sense of it all then just search for additional videos or tutorials that may address any remaining questions.
Facebook
Twitter
Instagram