First you need to call the dog by it's name to get it's attention. Show it the treat, say it's name again, say the command(Important: Make sure you always say the same command and use the same hand gesture every time!), and roll the dog over by hand. Get excited, tell the dog that (insert command) was good and reward the dog. Do this a few times in a row. If you keep it up the dog should learn that rolling over when you say command and use hand gesture equals treat. Eventually the dog could probably do it without you having a treat and just using the command and hand gesture.