re: "I've NEVER understood how to throw a screwball..."
---------It's easy enough to explain how to throw a screwball; it is quite another matter to throw one.
A typical screwball grip is somewhat similar your curveball grip, but it is pre-set and released with complete pronation (palm directed sideward, away from your body). Think of an "upside-down karate chop".
Another difference is, to get good topspin on the screwball your index finger will be resting along a seam (rather than your middle finger, as in a typical curve grip).
So, try this with a baseball in your hand:
Apply a typical 4-seam curveball grip. Rotate the ball about 1/8th of a turn in your hand...that is, just until the top side of your index finger rests up against a seam and your middle finger is on a smooth surface. (Note, your thumb and middle finger should "cut the ball in half").
Now, pronate your forearm/wrist/hand until the palm of your hand is facing to the side, away from your body. Again, think "upside-down karate chop".
That's the grip, now go out and see if you can throw one....if you can, you will have a pitch that shows about the same downward break as your curve and a lateral break that goes in the opposite direction of your curve. Back in the day, some LHPs worked on their screwballs so that they could reliably throw a breaking ball to the outside of the zone when facing the abundant RHHs in baseball....