Good advice in both of the above posts. My suggestions was simply a practical piece of advice, but as I said, if you want to learn then look carefully at the above 2 posts. When I was new at this, I would take a piece of paper and draw a rough gearbox shape, then poke holes where the screw holes on your gearbox are located, then put the screws from the gearbox in the appropriate holes in my template. Then you know you are putting the correct length screw in the holes when reassembling.
-- Example: Put a motor mount screw up where the selector plate is and watch the fun as the plate tries to slide over the 1mm of screw that is protruding through the gearbox.