Summarize the following:
First, arrange your hands into the basic grip for the pitch you want to modify. Modify it by bending your index and middle fingers at the first joint. Place your fingernails flat against the ball and keep these 2 middle knuckles raised up. When you release the ball, allow your top knuckles to push away the Wiffle ball to give it a spin. For example, with a knuckle slider, you'd get a counter-clockwise spin. Don't let your ring and pinky fingers wrap around the ball. Position your hands higher on the side with no holes for this pitch. For dramatic movement and a big break, turn your wrist slightly so that the holes face the batter when you follow through. Snap your wrist so your fingers graze the side of the ball when you release it. This will be a slower pitch than a regular slider but the dramatic curve will throw off the batter. This works best with a scuffed or cut Wiffle ball. Hold the ball with the holes as you would for a regular slider or riser, then insert the tip of your middle finger into 1 of the holes. Wrap your other fingers and thumb loosely around the ball. Throw a slider change-up over the top if you want to make the ball break left, or try throwing it at 3/4 if you want to add some rise toward the end. Use different arm angles to make the ball break at different spots when throwing a riser change-up. Face the ball's holes toward your palm and hold the ball with a secure grip. Just be sure not to cover the holes heavily with your fingers or palm. Hold the ball with just your index and middle fingers, or add your ring and pinky fingers to the grip to modify this pitch. Throw this forcibly as you would with a fastball, but whip the ball out of your hand at the end with a snap of your wrist. The goal of a change-up is to trick the batter into thinking you'll pitch a fastball, but the change-up will be a slower pitch.

Summary:
Bend your index and middle fingers against the ball for a knuckle pitch. Adjust your follow-through by snapping your wrist for a nasty slider. Place your middle finger into a hole for a slider change-up or riser change-up. Hold the holes toward your hand and use a loose grip for a straight change-up.