I learned how to take apart gearboxes mostly on my own. The first time I was a little scared, so I took pictures every time I got to a new step. Luckily I didn't have to use them, but once you understand how they work, it's almost common sense... Unless the engineers who designed them used 20 different types of screws and pieces that look virtually identical but don't work when interchanged :P. Probably the best way is to see someone else take it apart (maybe youtube) and when you do it yourself, you can fill in the blanks.
One problem you might encounter is some parts are attached to a spring, and when you open/unscrew something, something might fly off (I lost my hopup nub this way).