There are some great resources on how to set up and use OctoPrint. First of all, what is OctoPrint? Click here to learn all about OctoPrint from the official source.
To set it up with the Tevo Tornado is very similar to setting it up with any 3D printer.
This is a video that will help you get your OctoPrint setup, it covers all the basic configuration of the Raspberry Pi. Setup Video
When using OctoPrint for the first time, be sure to turn on the power switch for the printer. If you just plug the USB cable in it will come on but the heater will not work unless you turn the power switch on. You may get a communication failure along with a message asking you to reset the printer.
Once you have it setup, OctoPrint will let you control your printer remotely. If you include a webcam you will be able remotely view the printer as it's working.
One feature that is missing with OctoPrint is to hide the main screen from a guest (someone who has not logged in to your server) this means, if you port forward and expose your OctoPrint to the Internet (not a secure idea), anyone will be able to see your printer, view your webcam and download your time lapse files. It is important to consider security when doing this. - In researching this issue it appears that it is a major code effort for the folks at OctoPrint to fix.
If a print should mysteriously stop this could indicate a communication error between OctoPrint and the printer. Try going to the terminal tab, at the bottom click on Advanced Options, then click on the Force Fancy Functionality, then click on send Fake Acknowledgement. This may get your print working again.
You can set up one OctoPi to drive multiple printers. Each printer must have it's own OctoPrint instance, the Raspberry Pi 3 should have plenty of horse power to send g-code to multiple printers. Fortunately, Chris Riley has created a YouTube Video that explains how to create multiple instances. He creates four but you can easily change this if you want fewer. He did mention in his video that he 9 printers running on two Pis - That's pretty impressive.
For testing the USB settings there are some useful commands here