Terminal Svcs through a redhat firewall

I'm a newbie to Linux, but very familiar with NT/2000. So please tell me like you would a 3 year old.

I have a Linux firewall up and running using ipchains. It's also performing NAT for my internal network. I want to be able to allow Terminal Service sessions through the firewall to a server on my internal network. TS uses port 3389. The problem is the destination IP for the packets will be the firewall's external IP. How do I tell my firewall to forward or redirect incoming traffic over TCP 3389 to the internal IP address of my TS server?

Thanks in advance.