Summarize the following:
A new ball will swing naturally in a conventional direction, but at about this age, wear on ball changes its aerodynamics. It begins to swing in opposite direction of seam, towards the shine. Reverse swinging works best when the smooth side of the ball is very smooth, the rough side is very rough, and the seam is prominent. Continually polish the smooth side of the ball during play. Keep in mind, however, that scratching the rough side constitutes tampering, and is illegal. Grip it with your middle and index fingers on either side of the seam, with the ball resting on your thumb and third finger. The rough side should face the direction of the swing. This means the shiny side will now be facing away from the batsman. The difference between a conventional swing and a reverse swing is that in the former, the ball swings in the direction of the seam, while in the latter it swings in the opposite direction.  To bowl an inswing, release the ball with the seam pointed at about a 20 degree angle towards the slip fielders. The last point of contact with the ball should be your middle finger.  To bowl an outswing, release the ball with the seam pointed at about a 20 degree angle towards fine leg.The last point of contact with the ball should be your index finger.  Bowl hard. The faster you are able to bowl, the more effectively you will be able to achieve a reverse swing. The speed necessary also depends on the state of the ball; the rougher the rough side of the ball is, the less speed you will need.

Summary:
Use a ball 40 or more overs old. Keep the ball in good shape. Hold the ball along the seam. Bowl as you would a conventional swing, but with the sides of the ball reversed.