From your original description, I am unsure as to the actual problem. Others have concluded that your raster is simply rotated. If that's the case, then rotating the yoke will certainly fix it. If the raster has other geometric problems, then you'll have to do extra work.
In any case, assuming that a rotation will cure what ails you, follow Scott's basic procedure, with the following additional notes:
1) The yoke is glued to the crt, so simply loosening the screws will generally not allow free rotation. You will need to grab the yoke and twist steadily and firmly until it breaks free. You will likely feel nervous, but don't worry.
2) Do not simply turn off the Mac; pull out the power cord. As long as you follow this rule, it will be safe for you to stick your mitts inside.
3) When you are satisfied that the yoke is properly adjusted, tighten the yoke's retaining collar. Do not overtighten the screw, or you will break the crt. Tighten the screw only enough to prevent free rotation of the yoke. Once that's been achieved, you're done.
If you need to do more than adjust rotation, see http://68kmla.org/files/classicmac2.pdf for additional notes on what to do.