First task is to get some finer shims than the ones in the kit. Yokomo do some, I used the ones from TOP. 12mm for the diff, 8mm for the input.
Then, shim out the excess play on the input gear, and line it up so that it meshes in the middle of the diff gear. Also make sure that the input gear spins freely in both gearbox halves (mouldings are not necessarily identical).
Then, work out how many shims the diff can carry before it stops spinning freely in both gearbox halves. Now adjust those shims until you get a setting with the tiniest amount of backlash. If you have no backlash, make sure the gears turn smoothly by hand, no grinding.
Once assembled, check that everything is still free running with minimal backlash (fingers on the diff outdrive and the centre driveshaft will do this), and run the car conservatively for 5 minutes to bed the gears in.
Recheck the mesh, adjust again if necessary. That should get it right.
I had big problems getting the mesh right on the car. The two biggest problems were the thick kit shims (at 0.2mm, too big for an accurate adjustment) and the unequal moulding of the top and bottom gearboxes, (which would make a mesh that seemed OK in one half be too tight in the other).
Hope that helps.
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