Port Forwarding makes all traffic using that port routed to one specific server. In your case you want all traffic on the port (21 for ftp) to go to your internal IP. You just need to go into your router, where ever your port forwarding options are (under applications and gaming for lynsys routers) and forward port 21 to your IP, (i recommend switching to a static IP rather than a dynamic if you have the router automatically assign IP addresses)