I'm not the worlds best but I can lay down a decent run with it, so what I do is using the side of my thumb flick down onto your E n A strings while plucking(popping) with my forefingers the D n G strings. hope this is some help.
Hi here is a good video tutorial by the legendary Tony Saunders.
Hope i helped...
try to position your hand so that your fingers are pointing to the head of the bass, this may take some getting used to but it helps. then using the bony joint part of your thumbquickly strike the E or A string and quickly bring it back up, to pop leave your hand with the fingers pointing to the headstock and make a thumbs up figure without your fingers touching your palm. now slap a string and then using your pointer finger get under the D or G string and pull up. after a while you'll be able to put them both together There's also a free slap bass tutorial online available at HowToSlapBass.com