Assuming you already know how to access your router's control panel (192.168.1.1 in your browser) you go to Firewall Settings and select Port Forwarding on the left menu.
Next, select Add, and it will take you to an Add Port Forwarding Rule box.
In the drop down Networked Computer / Device: menu where it says "Specify Address" select the designation that corresponds to your FTP server. (Mine would be COMPUTER-6.)
Protocol: select FTP.
WAN Connection Type: All Broadband Devices
Forward to Port: Same as Incoming Port
When should this rule occur ? Always
Hit Apply as needed.
That should do it. 
Now your people can reach and access your FTP files by typing ftp://xx.xxx.xx.xxx (your outside IP)
Now of course, you need to have FTP Server already set up on your system, but I assume you already know that. A couple of caveats here. Your outside IP Address can change, or it may stay the same for a year. That all depends on Verizon and if they had to break your lease for some reason, or there was a problem that they had to reset your outside IP. So you need to keep an eye on it. (It appears on your login page in your router's control panel.)
Then too, if your router changes your FTP server's IP addy for some reason, you will maybe need to change the inside IP designation in your FTP software settings. Depends. All I know is the Windows IIS version, and that needs keeping an eye on, to be sure.