Before you do anything, open the app and go to its Settings.
Here set the username, password and port.
Also, set the default directory on the device that you want the server to present to FTP clients.
Make sure you carefully configure the Allowed connections section carefully to restrict access only from within certain networks, like your home wireless network for example.
Once configuration is done, choose Save and restart service.
Whenever you want to connect to the device over FTP, run the FTPServer app and choose Start FTP service. You should now be able to connect from a computer on your network to the FTP server on your Android device over FTP.
Tried with: FTPServer 2.2.1 and CyanogenMod 10-20121228-NIGHTLY-encore