Linksys Wireless Router :: WRT54GS - Localhost Test Site?

Have a localhost site to test PayPal IPN which apparently requires router configuration to use port forwarding (or redirection) to send TCP traffic on public port 80 to my test machines private IP at a specified port.
 
A typical configuration might look like this:

Router NAT Port Redirection Rule TCP Public Port 80 --> 192.168.1.2 (Port 8888)
 
Where can I find reference to this in online documentation and where do I put this instruction?
 
My Router is  WRT54GS