[Internet-available FTP server on my LAN, how-to]

You must use the router control panel Port Forwarding feature to forward Port 20 & 21 requjests to the ip of the computer that has the ftp server.

However, keep in mind that this may not work because:
1. many isps filter port 20 & 21 because their terms of service agreement prohibits the use of ftp servers for non business internet accounts.
2. a software firewall on the computer that has the ftp server may block ftp by default and must be configured to allow the ftp service or ftp server application.

The solution to #1 is to configure the ftp server to use ports other than 20 & 21 and setup Port Forwarding to work with those ports. And make sure the ftp server is secured else you will get unauthorized access to your computer.

But, prior to doing any of the above you should locate the terms of service agrement of your isp, it's usually online somewhere at their website. ISPs, esp broadband isps, regularly scan their network of users to see if ftp servers are being used and if they catch you then they will take action, usually by cutting off your service or reducing your bandwidth.

It's not worth the risk and is better to just use email to transfer files or to use some other file sharing application such as messenger apps like icq, aim, etc.