Plug the guitar into an amplifier.
Turn on the amp and set the volume to the level you normally use.
Turn the volume dial down to zero to keep the noise of your screwdriver from being amplified.
Tighten the screw on either side of the humbucker pickup cavity by turning it in a clockwise direction. This raises the pickup toward the string.
Tighten the other screw by an equal amount so that all strings have the same output level.
Turn the volume dial up and play the guitar. It will be louder than before the adjustment. If the strings are rattling against the pickup, turn both screws counterclockwise by an equal amount to lower the pickup. If there is room and you want to do so, continue turning in a clockwise direction.
Loosen the screws on either side of the pickup cavity.
Turn the volume dial up. Play a riff that you frequently use, which will make it easier to asses the change in tone.
Flick the pickup selector switch to engage another pickup. Compare the volume of that pickup with the adjusted humbucker. If there is large volume dip when the humbucker is engaged, tighten the screws.
Tighten one of the screws. Tightening the screw on the treble side of the pickup will raise that side.
Loosen the screw on the opposite side to lower the other side of the pickup.
Turn the volume up and test the guitar. The thicker strings should be quieter than the top strings. To make the thinner strings quieter, perform the reverse screw adjustment.