It's pretty easy. You'll need 3 things..
1. Sign up for a free dynamic ip (i.e. Gremlinhater.xxxx.org) etc.
2. Download Serv- U
3. Download Apache 2.0.40
Set the 2 programs up with the free dynamic ip as the host. Then in Serv U, make a user, configure the port that you want to use (if not 21), and enter a user/ password, how many users, etc and show it which folder you want to use as the FTP folder. I would suggest making 1 directory (you can call it FTP) and then 2 folders in it. 1 for downloads and 1 for uploads. That way you can set preferences for both of them that people can download only from download and upload only to upload folder, etc. 🙂