I'll try my best to explain
Basically, take all the shims off the arm, put it back in the can (with the brushes out), and give it a spin.
When it comes to rest, it should be sitting in the middle of the field.
Then give it a push and a pull to feel which end needs some shims.
Fit shims, and repeat, until you end up with an arm that sits in the middle of the field, with only a tiny amount of play each way.