Any good tips on how to do these? i know you have to hit the string with your thumb as you pick, but it just doesn't work too well for me, any help?
turn up the gain.

make sure theres not too much pick showing (i.e only the tip)

it needs practice practice practice. i doesn't tend to come naturally for most players.

you gotta 'flick' sorta. thats how i do it. main thing is practice.