When slappinig do i use my thumb to hit the string against the fret board or do i go up and down like using a pick. I use my thumb to hit the string onto the fret board but after waching a bit of a tutorial vid it showd him slapping up and down, is this correct or are both ways correct and its down to preferance?

The method of slapping is just like you said- hitting the string with your thumb against the fret board.
picking up and down with your thumb is a different technique called double thumping. victor wooten uses it a lot.