How to master center ball:
Place the CB on the headspot. Aim at the far end rail at the center diamond. Roll the CB up table leaving your cue extended through the CB, hit the rail at the center diamond and have the CB roll back down table and have it hit your unmoved cue on the tip.
Do not stop this excersize until you can do this 10 times in a row.
If the CB does not hit the end rail at the center diamond, it is likely that your back hand (the one holding the cue) moved on the back or fore stroke. You must get to the point you can hit what you aim at--or all is lost.
If the CB rolls up table and hits the rail at center diamond but does not come back down table, you have hit the CB on the side and imparted side english. You must get to the point you can hit the CB along the vertical point (no side-spin) or you will never get control.
One thing a lot of beginners think/believe is that you need to hit the CB with the cue-tip. This is absolutely FALSE. What you need to do is to deliver the cue-tip to the cue-ball at a precise point. Watch any player on TV, notice how deliberate they are with the stroke, and notice how deliberate they are with the cue tip. This is not like baseball where you hit the ball with the bat and hope it goes somewhere useful. This is pool where you have to deliver the cue-tip to the cue-ball precisely, deliberately, and with delicasy. Being able to deliver the cue-tip to the cue-ball with speed and delicasy is what separates the good players from the also-rans.
One other thing beginners (even into intermediates) believe is that you have to hit the CB (or OB) hard. This, too, is absolutely FALSE. You want to develop the "Feel" of the movement on the Cue such that you can roll the CB one diamond, 2 diamonds, 3 diamonds, 1 inch,... Later this will translate into being able to roll the CB down table, pop the OB into the pocket, and leave the CB right where it touched the OB. Thus, without any english, you avoid a scratch. Later on this feel will alow you to manover the CB around table obtaining position or placing the CB where the opponent has no shot.
And as long as we are on the basic hitting of the cue ball topic:
One excellent excersize is to place an OB on headspot and place the CB one diamond behind the OB in a straight line to the downtable corner pocket. Hit the CB into the OB and make the shot. When you make 10 in a row, move the OB one diamond down table, but leave CB one diamond from the end rail. Any time yo make 10 in a row, move the OB farther down table. Soon you will see that the hardest straight shot is when the OB and CB are just over 1/2 table distance apart. After a few weeks, you will be able to make any long straight shot and never get nervous again over these (easy) shots.
Once you have mastered the straight shot at all distances (still using no english (spin)) use the diamonds to place and OB at the intersection of a side rail diamond and an end rail diamond, place the OB at some othe side rail and end rail diamond and shoot that shot until you can make it 10 times in a row. After a week of practiing this excersize, try to roll the OB into the pocket so that if just barely makes it to the pocket and holds for a second on th eedge before dropping in. Stay down on the shot, and watch the CB. This excersize will develop your eye to the angle you need and if you are practicing the delicasy mentioned above, you will realize that the CB moves to the side exactly the same distance seen down the line of aim as the OB moves to the other side. We will use this observation later when we actively try to position the CB for the next shot.