you really only need to use half of the tip (the "third") of your index. Your arm should be at an angle.....here is a picture so it makes it easier
see how my arm is at an angle to the strings instead of horizontal (i dont know if that is one of your problems)
And also, you should keep your hand like mine (see how the fingers are just automatically curled? kind of like a monkey, haha. This makes it simple to pop the string quickly. You should just practice that while walking around or something. Just atleast get the motions down.
Try not to be agressive too. just slap in a "laid back" manner, instead of RIPPING at the string, you know?
All you really need is small movements so it is easier to just go right into the next thing you want to do
I hope this helped
