im not familiar with winproxy, and you dont say what ftp server software you are using, basically you need to forward ports 20 and 21 from the system with the direct net connecton to the system running the ftp server. The ftp server software will probably need passive mode enabling and any ports other than 20/21 that you want to use putting in there and also forwarding, and setting to your internal ip.
If they squeeze olives to get olive oil, how do they get baby oil?