Keep your thumb to one side, play some bass notes and play something else with your other fingers on the other strings. That’s all there is to it. The alternating index-middle technique in Derek’s
video is a good starting place and just one thing you could try. This
exercise is a little harder, as it combines thumb, index and middle, which might be what you’re looking for.
You can use just about any exercise or simple arrangement for practice, or just make up a pattern that you can commit to memory. Spend 10 hours a week playing that and it won’t be long before it feels more natural.
One of my favorite guitarists kept his thumb behind the other fingers, but I wouldn’t recommend trying to play that way.