Every router is different. You will probably have to log into your router's web interface. It's common (though not guaranteed) that your router might reside at 192.168.1.1, or 192.168.100.1, or 192.168.10.1.
Once you log into your router, you'll have to find the port forwarding configuration screen. Then you'll forward ports 20 and 21 to the IP address of your Linkstation, and open those two ports in the router's firewall.
Frankly, if all this is too complex for you, I wouldn't suggest doing it at all. The fact is that when you start opening ports on your firewall, you expose part or all of your LAN to potential abuse from the outside world.
For example, let's say you manage to set up your router correctly, and set up the LinkStation to handle FTP services. But you fail to properly restrict access to the FTP available share. Well, then someone on the outside may be able to read your files, or worse, delete, alter, or add files to your drive without your authorization.
However, if you are able to configure your router correctly, and the linkstation correctly in every way, the LinkStation's FTP services are convenient and secure.