Need help to setup my file server

Question

I've got this old G3 which I need to setup in the office so it serves files for remote work from home. The G3 is behind a DSL modem (Alcatel SpeedTouch 530i) which connects by Ethernet cable to my AirPort Extreme Base Station. G3 picks up wi-fi signal from AEBS.
From outside the office I need to be able to go to an internet address and log into my G3 and work on a file, then save the changes back on the G3 for other colleagues to do the same.
I understand that I have to do some port-forwarding, both on the Alcatel and on the AEBS, only I don't know the details. Could some genious help? I am good at following instructions, by the way.
Many thanks, guys.

Answer 1

I take it that I need to forward port number 20. Or is it 21, as well? The G3 will host work files which colleagues will have to have access to from over the internet.
I changed the G3's TCP/ID addy from DHCP to manual, and entered the one that was assigned to it anyway (10.0.1.2 on subnet 255.255.255.0).
Router addy has remained the same (10.0.1.1).
Then I went into my Alcatel's config page in Safari, and clicked Advanced, then NAPT. I clicked on 'New', then was presented with the following options to setup:
Protocol: TCP or UDP;
Inside IP;
Outside IP;
Inside port;
Outside port.
The Alcatel modem's address is (as shown in its config page's SystemInfo window): X1.X2.X3.X4/32 (Xs stand for actual numbers of course).
What do I do next? Do I set for the New Port Forwarding as follows?
Protocol: TCP
Inside IP: 10.0.1.2
Outside IP: X1.X2.X3.X4
Inside port: 20
Outside port: 20
Please bear with me. I can learn. Many thanks.