Here is the basic format to forward ports:
ip nat inside source static tcp
interface
For example:
ip nat inside source static tcp 192.168.0.10 80 interface Serial0/0/0 80
This forwards port 80 to 192.168.0.10. From the Internet you would connect to the IP address assigned to the Serial0/0/0 interface on port 80 to reach the 192.168.0.10 host. Substitute Serial0/0/0 with the interface on the router connected to the ISP (the one with "ip nat outside" under it).
You may or may not have to modify an access-list on the router. That same WAN or "ip nat outside" interface may or may not have "ip access-group in" under it. If it does, you will need to add a line to that access-list in this format:
ip access-list ext
permit tcp any any eq 80
This is for the port 80 example.
