When thrown, try to manipulate the pitch to come off of the thumb-side of your index finger – NOT your index- and middle-fingers, as with a two-seam fastball – because a two-finger release will cause the pitch to balance out, which reduces the spin that you are looking for. Most good slider pitchers grip the outer-third of the baseball and cock their wrist slightly, but not stiffly, to their throwing hand's thumb-side upon release of the pitch. This enables a pitcher to apply pressure to the outer-half of the ball with the index finger. Avoid any twisting of the wrist upon release.
The beginners curveball is a great pitch for younger pitchers. In essence, this pitch does the exact opposite as a fastball. Where as a fastball spins from the bottom to top (which is known as "backspin"), a curveball spins from top to bottom. And instead of leverage coming from behind the top of the baseball (as a four-seam fastball), leverage on a curve comes from the front of the baseball.
In this video I answer some questions about the King of the Hill Pitcher’s Trainer, one of which was “what type of pitching drills can you do on the King of the Hill?”. If you like the King of the Hill, and I definitely suggest this product for pitchers of all ages, then you can check it out at King of the Hill. I really love this product because it teaches pitchers how to properly and efficiently get the most energy out of their legs.