How to make a cricket ball swing
The standard method used to make a cricket ball swing is to make on side shiny and other other side rough. The bowler delivers the ball with the stitches (seam) held upright and pointing in the direction they will bowl. The rough side is held facing the side of the wicket the bowler wishes to make the ball swing towards.
For example, to bowl an outswinger, where the ball swings away from the batsman to the slips, the bowler holds the ball with the seam pointing to the slips and the rough side of the ball on the slips-side of the wicket.
Why does a cricket ball swing?
The different textures of the two sides of the ball force air to pass over the two halves at different speeds. Air moves slowly over the rough side of the ball (high pressure) and quickly over the shiny side (low pressure), making an air pressure difference that causes the net force to push the ball towards the rough side.
The weather can also affect the amount of movement of the ball. On humid cloudy days when there is little wind, the ball swings more. The theory is that the humid air is heavier and accentuates the effects of the rough and smooth sides of the ball.
The technique of the bowler also seems to have an effect. It seems that the bowlers with a more 'side-on' action find it easier to bowl an outswinger, and those with a 'front-on' action find it easier to bowl an inswinger.