Stagger,
the firewall on a router like this is more a byproduct of the NAT, than a proper firewall. So there is only one step in setting this up - forwarding the relevant port to a local IP.
Unfortunately for zoink, according to:
http://www.fulton.net.au/dm602.htm,
"The DM602 in router mode only supports DMZ, not individual port forwarding. Use a DG814 instead, or use the DM602 in modem mode in conjunction with another router such as the FR114P, FM114P or FVS318 if you require individual port forwarding."
"DMZ" means forwarding *all* ports to a particular IP - equivalent to completely exposing it to the net. Not something I would recommend.
This has me slightly puzzled (why Netgear should leave out a router feature that I regard as essential) so I've looked at the manual at netgear.com and although the section on DMZ mentions "the port forwarding menu" I find no other reference to it.