It's a piece of cake, just remember that the outside beveled edge of the two gears should be perfectly aligned if you place a straightedge across...they are machined that way for a reason. Most mistakes I see are because someone forgot that simple rule and installed the gears and ended up putting in shims and getting way off track trying to fit the gears by pattern instead of by backlash. The cases are precision machined and should need either no shims or only one, if your stacking there is something wrong with the case or your setup.
Little hint...if the gears line up well and have good backlash but you are still having trouble getting a good pattern this usually means the case is spread and the pinion is out of line. I would just get another case if you find one like this, it's not that common but you see the odd one.
Also stay away from the "drag race only" gears, they are cut biased towards the heel and are designed to flex under load before the pattern looks right and can really mess with your mind if you didn't know that while your setting them up.
Have fun!