Banking can be an elusive art for some folks, but once you start to practice it you'll find it really isn't as mistical as people make it seem.
For banking I try to use the equal distance banking method. The equal distance banking method suggests that you need to aim at the half way point between where the object ball is and the pocket you are are trying to bank into. So in the diagram below the cue ball is 4 diamonds away from the pocket so this means we need to take half of that number and aim at the second diamond in order to make the bank shot.
There are some factors that can affect whether this system is effective or not due to the physics on the table. Because the cushions are made out of rubber they will not bounce the cue ball back at a perfect angle every time. If you hit the ball hard the rail will compress and the angle it comes back will be shallow. If you hit the object ball soft then it will come off the rail at a larger angle then expected. Getting used to using medium speed takes some practice, but knowing when to compress the rail can be helpful in tough situations.
Another factor of banking is when you put unwanted spin on the cue ball. When you do this some of that spin gets transferred onto the object ball which can alter the direction as well. Spin can also result from cutting the object ball if the bank isn't a straight on bank. When you cut a ball spin is placed on it and this will greatly affect where you need to hit a bank. Most people don't talk about this factor, but you need to keep it in mind if you ever have a bank that isn't straight.