How do I teach my dog to roll over?

My dog is a dachshund beagle mix and is about 6–7 years old in human years. He has never went to training school. He learned how to sit when he was probably about 3 human years old and I taught him how to lie down about 5 months ago. He isn’t the smartest dog, but if you teach him correctly with a gentle voice, he will learn quickly. But he is mentally sensitive so if I be a little strict on him, he’ll try to walk away. I want him to know how to do more tricks. So, do you know a good, easy way to teach him how to roll over?

p.s. If you want to know how I got him to learn ‘lie down’, ask me and I’ll tell you! (But it might not work on large dogs.)