[header] How to throw a spitball [title] Put a small amount of saliva or vaseline on the baseball. [step] The object is to create resistance and a change in the uniform weight of the ball to make its motion more unpredictable and difficult to hit. There's no " right place " to put it, since the object is unpredictability, as with a knuckleball. 
[substeps] Alternatively, scratching the ball with a file or sandpaper can be a similarly-effective way of messing with the weight and the trajectory. It's still illegal, but it's a possibility.