Under most circumstances, you should never need to touch the yoke of your CRT. Once tweaked it can be very difficult to align the yoke properly so that you get good geometry, convergence and purity. That said, there are two situations in which you might need to adjust the yoke on your CRT. Note that you must do these adjustments with the CRT on and displaying a picture so that you can see the effects of your adjustment directly. Invest in a good pair of lineman gloves so that you can safely grasp the yoke to make adjustments. Note also that some yokes are held in place by two set screws instead of one.
The recommended picture to display on your CRT while making these adjustments is a cross hatch grid pattern. This will allow you to clearly see the rotation, geometry and convergence while you are making the adjustments. Also, it is recommended to use a cross hatch that colors the outside edges with red and the center with pure white. This is so that you can see if you have messed with the purity while adjusting the yoke. If you have an all white cross hatch pattern it can be difficult to see if you've screwed up the purity because the shifted colors will still sum up to white. Using a pattern with red borders will allow you to quickly see purity issues while still giving you a white pattern in most of the screen to judge convergence. You can find such a test pattern in the 240p test suite or in the naomidiag test suite.
The first problem that can be fixed with a yoke adjustment is a rotated picture. If your picture has good geometry and convergence but appears rotated a few degrees clockwise or counterclockwise, then your yoke needs to be rotated. Loosen the set screw holding your yoke enough to be able to twist it back and forth, but not enough for it to slide backwards down the neck. Then once you've gotten the picture level, tighten the set screw. Be careful not to tweak the yoke left, right, up or down as this could affect convergence and geometry. Note that the wedges between your tube and yoke might be glued to the yoke itself so this can make adjustments a bit fussy.
The second problem that can be fixed with a yoke adjustment is geometry, convergence and purity issues. If your monitor appears to sag on the left and right, has weird curves on one side of the picture, has incredibly poor convergence or even has purity issues, a yoke reseat can help. Loosen the set screw holding your yoke enough to slide it forward and back and snug it up against the wedges of your tube. With any luck, this should resolve any geometry or purity issues you are experiencing, or at least make them better. Under very dire circumstances you may want to pull out the wedges, reposition the yoke for better purity/convergence/geometry and then reinsert them once you have tightened the yoke set screw down again.
Be warned, repositioning a yoke without the factory wedges is very time consuming and difficult. It may take hours of fine tweaking to get it just right and you may very well make things worse.