Depends on your monitor.
There are 3 controls you will have to fine tune. First off, there is the screen control on the flyback. Odds are this will fix your problem. Second, you will want to fine tune the brightness and contrast pots on the back of the chassis. In most cases, if you turn all of the pots to exact center they will be right. Then you just dial in the flyback screen control and adjust the flyback focus knob.
You need to get the model number of your monitor chassis, then get a pdf manual for it online. There will be a diagram in the manual showing the locations of all the adjustment knobs.