[Styrak]

There is no specific way to shim a gearbox. Every gearbox is different.
If you want to be sure it's shimmed correctly, take out the ant-reversal latch, cylinder, cylinder head, and tappet plate. This will allow the gears to spin freely. If they can smoothly make a few rotations when you spin them with your finger with the gearbox screwed down (through the hole previously occupied by the cylinder) then it should be fine