there are a couple of ways you can go about this problem.
Some people will use their wrist and forearm, barely moving their fingers, and pulling off so their fingers just pluck once after the other.
The other way, which i prefer becuase you get better control, and economy of motion. is to keep your wrist very still and then use your fingers to do all the work. This will probably mean you need to strengthen your fingers a bit. But don't think that just because it's called 'slap' you have to play hard. Plucking hard menas the string is moving around alot more, so double plucking the same string and keeping it rythmically tight becomes alot harder. If your finding this a problem, play a bit more gentle, and roll the volume up a little.
