The video is a bit long-winded, but the info is pretty great.
- Switch the AF mode to AF Continuous, i.e. focus follows subject as long as AF button is depressed.
- Use the AF On button if your camera has one, or reassign the AF Lock/AE Lock button to be AF On. I.e. pressing the button engages AF.
So, now, this will be a little weird, but focusing is now done by depressing the AF On button with the thumb as opposed the partially depressing the shutter release.