When you open the app (I did it on my MacBook), it will read the iTunes information on your computer. Within the app, select a playlist(s), select the destination folder (your usb drive), and hit export/start. You can set it to just copy the playlist info (if you've already copied everything to the usb), or you can set it to copy the playlist info and the music files.
When I tried simply exporting the playlist to the usb, it wouldn't work because the computer copies the source file path, and not the usb file path. There is a way you can do it manually, but I decided the cost of the app was worth it to make it easier.
I'm on a iPad. No USB port. There is probably a PC app that does the same thing....or a magical device that allows you to connect a USB to the iPad.