I made this small gearbox for use in RC boats, where I want to use a brushless motor, but need to slow it down, as my propellers' efficient speeds are much lower than my motors' efficient speeds.  The gearbox will work for most small motors with a 25mm bolt spacing.  Obviously, one must purchase a set of gears that have the same pitch as each other, one small one for the motor shaft, and one large one for the propeller shaft.  I use 48P because a lot of my friends use that pitch, but you could use nearly any pitch.  

  The basic assembly is easy;  Fettle the output shaft stuffing tube (or bearing) hole to fit your stuffing tube (or bearing).  There are two files, one for a 1/4" (6.4mm) stuffing tube, and one for a 5/16" (8mm) one.  Gently slide the stuffing tube into the fettled hole.  You may wish to secure it with CA glue, or by smearing a slurry of ABS scrap and acetone onto the stuffing tube before inserting it into the gearbox.  The stuffing tube end should not protrude into the gearbox more than a couple of mm.

  Put the driveshaft into the stuffing tube and put a brass collar and then your large gear onto it.  Mark it for flats, then pull the shaft out and put the flats on it.  This is simple, but crucial to having everything work under load, so I'm saying it plainly :)   With that done, reinsert the shaft into the stuffing tube and put the collar and then the large gear on it.

Fit the small gear to your motor's shaft (don't forget to put a flat on the shaft!), and fit (but don't tighten) two M3x8mm screws to your motor's mounting holes.  If your motor uses larger screws, you may need to fettle the ways a bit.   Align the screws with the ways (the vertical channels) and slide the motor down until the gears almost mesh. 

Slide a piece of printer paper between the gears, and move the motor down until the gears are in good contact.  Tighten the screws, and you're good to go.  spin the motor by hand and the paper will come out.  The point of the paper was to make sure that your gears weren't too close together for good operation.  This reduces wear and noise.

Happy boating (or whatever you're doing with the gearbox!)