Its one of those things like riding a bike, piss easy once you get it.
Practice practice practice, took me 3 days to crack it and be 100%. It is super useful.
Can do any finger and thumb. Left hand harder
Just reduce the gap between thumb and finger til it almost touches. Try and breath air through, not blow..I pull bottom lip tight across bottom teeth, the lip rolls over them slightly. Fingers rest on bottom lip I tend to use thumb and middle finger
The two hand option is the same principle but you look like just William so give it a swerve