It's pretty straight forward. The only advice I would give is that the shaft going through it for the tires isn't one piece, each tire has it's own shaft. So what I usually like to do is to put one in, and put the tire actually on it, then slowly push it in (probably with a hammer at this point) until the tire is dead center on the railing. Then by having one of them in place exactly where it needs to be, it makes putting the other one in a no brainer, as you just push it in as far as it can go until it bottoms out on the other shaft.
Don't forget to take the breather off of the old gearbox and put it on the new one, as the new ones apparently don't come with this piece, it's a separate part.
edit: I just realized you may have been talking about the arch rotate gearbox, not the trolley gearbox.
