First of all it’s important to note that the
spitball is an illegal pitch. Any pitch that places a foreign substance on the ball or
defaces the ball or scratches it in any way is an illegal pitch, and the player is subject
to ejection if he is caught placing a foreign substance or marking up the ball in any way.
But the basic way to throw a spitter is you place a foreign substance on the ball whether
it be spittle from the mouth, Vaseline that you’ve put on the bill of the cap, or an emery
board that you’ve hidden in your back pocket. All of those can be used to deface the ball
to create a spot on the ball that makes it heavier on one side and creates odd movement.
But I’d like to again note, that a spitball is an illegal pitch.