http://www.21stcenturyairsoft.com/21cashimguide.php
You're welcome.
Keep in mind that this is a good start, but due to wear and tear (among other things), you may still require a knowledgeable person to adjust them.
You gears, once shimmed, should have a minimum of side-to-side movement within the bushings. To test you setup, install the bushings, shims, and gears (ONLY those parts), then tighten up the mechbox. If you can stick your finger in the mechbox and spin the gears easily, that's good. If you can't spin them easily or they don't continue to spin, you have shimmed too tight and will need to move/adjust your shims.
The other thing to check is the lateral movement in the gears. While the mechbox is still sealed with the gears, shims, and bushings installed, use a screwdriver or punch to push on the gear axles (the part you can see in the middle of the bushing. They shouldn't move too much from side-to-side...1 or 2mm only.