How to Throw a Forkball

STEP:
Start with your index and middle fingers along the seams like a two-seam fast-ball. Spread your fingers out as far as possible around the ball. The ball should be jammed further between your index and middle fingers than in a split-fingered fastball.
When releasing the ball, snap your wrist. This will cause the ball to have a slight forward spin instead of a backspin. The forward spin is what causes the ball to tumble down the strike zone.

Tips:

Try to not throw too hard, location is what matters.
Make sure someone is watching you as you practice, they may be able to give you some tips.
Always stretch before pitching or injury may occur.
When throwing the forkball on a right handed batter throw it on the 'high and in' zone
When throwing the forkball, be sure to come straight over with your arm - don't spin your wrist as you would with a curveball. The ball should drop dead from the forward motion.
Spend about a month stretching the tendon in between your index and middle finger before starting to seriously throw this pitch.

Warnings
The forkball is notorious for damaging the shoulder and elbow, so be careful as to not stress your arm too much. Young people especially should not throw many repetitions of this pitch.
Young people should pitch easier than normal while throwing this ball due to the strain on the arm and shoulder area.