This is the most common, and usually easiest to comprehend, method of aim training in pool. To employ the ghost ball method, simply visualize a line through the middle of the object ball to the pocket center. Then put an imaginary (ghost) cue ball at the spot where the actual cue ball must arrive behind the object ball in order to pocket it. It won't be long before you will automatically line up correctly without having to consciously visualize the ghost ball. Your memory takes over, knowing that the cue ball must arrive in that exact spot.
Once you've mastered the basics, everything, and I mean everything, in pool comes down to speed control. Pool has often been referred to as a game of inches, and for good reason. Speed-control and a well-played safety can guarantee you ball in hand on your next shot, but bad speed-control can force you to give up the table to your opponent to run the table instead.
Cue Ball Control
What you may not have noticed are the countless number of options you have, on each and every shot, to make the cue ball head in a direction that will give you the fundamentals of cue-ball-control consist of speed control, mastering center ball, and the use of follow, draw, and English. The combinations of all these elements, and how each of these affects speed control, themselves, are endless an easier attempt on your next shot.