Summarize:

Consistent stroke form and aim should be your first priorities when you start to take pool seriously. English is a very useful technique, but it has complex effects and you need consistency to practice it. You'll have trouble narrowing down the effects of English (side spin) if you're not also controlling the amount of overspin and slipping. These effects are determined by the height you strike on the cue ball. Slipping is completely eliminated at 2/5 of the distance between the center and the top of the ball, but in practical terms 1/5 of this distance is often a better measure for optimal control and speed. As long as there is no English, the cue ball will come to a dead stop after a perfect head-on collision. Practice head-on collisions striking the ball with your cue at the midpoint of its horizontal axis. Once you can get the cue ball to stop dead every time, you have enough control to introduce English to your game. There are several types of English, but this article sticks to the most basic form. If your cue hits the cue ball left of center, the ball will spin along this axis — this is "left English." When this spinning ball strikes a surface, the spin will cause it to rebound further to the left than a ball with no English. Similarly, striking the right side imparts "right English" and moves rebounds further to the right. The further from center you are, the more dramatic this effect:  100% English or maximum English means you strike halfway between the center and the edge of the ball. This is the farthest from the center you can strike and reliably avoid miscues. 50% English means you strike halfway between the maximum point and the center (¼ of the way from the center to the edge of the ball). You can use any other percentage of English by striking at different points between the center and the maximum point. When two balls collide, the object ball starts rotating around a particular axis, determined by the angle and the amount of English. If you achieve "gearing," this rotation occurs along the axis of movement. In other words, the object ball's motion is not affected by spin. It will travel exactly along the "line of centers," or the line drawn between the centers of the two balls at the moment of impact. The term comes from the analogy of two gears meshing smoothly together, transferring the motion perfectly. Once you've aimed your angle shot using the fullness or "ghost ball" approaches from the last section, you'd like to ensure that the object ball doesn't pick up any funny spin and ruin your shot. Here's where a chart can save you a lot of trial and error. All numbers below are for "outside English," meaning you move the cue to the side of the cue ball farther from the object ball.  If the cut angle is 15º, use slightly more than 20% English. (Remember, the cut angle is the angle between the cue ball's original path and the path of the object ball.) If the cut angle is 30º, use 40% English. If the cut angle is 45º, use about 55% English. If the cut angle is 60º, use about 70% English. As the cut angle approaches 90º, increase English to 80%. If you use less English than the "gearing" amount listed in the last step, the cue ball will slide forward during the collision, transferring side spin to the object ball. The object ball will move slightly to the right of the expected cut angle. If you use more English than the gearing amount, the object ball will move slightly to the left of the expected cut angle instead.  This effect is called cut induced throw: the cut angle transferred a spin which threw the ball off the expected path. You can use this to your advantage to make seemingly impossible shots. If your only clear shot would put the ball slightly too far to the right, increase the amount of outside English to throw the ball into the pocket.
Perfect your stroke first. Avoid English when in danger of sinking the cue ball. Practice different amounts of English. Understand gearing. Adjust your English to achieve gearing for any cut. Know the effects of a collision without gearing.