When you slap, it is actually two motions combined into one fluid motion.
First: Slap down roughly 45 degrees onto the string (the piak sound)
Second: Pull the string down as your thumb goes to rest on the string below it (the body of the sound)
So if you do it in slow motion, you should hear something like: piak...dooom. piak...dooom. And when you combine it into one full motion, you get a nice solid slap sound.
GO SLOW. With a metronome. and after that, GO SLOW. Always start slow. No one was born a Wooten.