Answer:
Check out the 'Throwing a Screwball' link on this page to find out about the mechanics of throwing a screwball.
Since there seems there isn't a link i will explain it for you. I used to throw it a lot. But what you do is you hold it like a four-seam fastball. What you have to do after this is you must pronate your wrist. Which means you twist you hand, wrist, and forearm counter-clock wise. This will give the ball a spin that will "chop" the air to the right of it (if its a RHP, and vis-versa if its a LHP) causing it to move that direction. But beware, it is a potentially harmful pitch, especially if done wrong! Use it sparingly! If you feel pain, then stop throwing it!