making a gearbox with an output shaft

1) find a motor and a set of gears to use
2) arrange the gears how you want them to be in the gearbox
3) find two metal plates big enough to encompas all your gears with room for putting screws through the box without hittings the gears
4) in two 'corners' of you plates, where there will be no gears, mark and drill holes through the two plates big enough for machine screws, drill them at the same time so they will be in the same place.
5) bolt the two plates togethor. Leave them bolted togethror for remaining steps
6) place your gears on the plates so that they mesh just right and mark the middles of the gears REALLY carefully on the top plate.
7) take off the gears, carefully centrepunch where the middle of the holes need to be.
8) put the gears back on the plate, with the middles of them centered on the punch marks and check the alignment
9) adjust the punch marks if needed and repeat the check till it all looks good. Remember not to have the gears too tight or they will generate of lot of resistance.
10) drill small holes on the punch marks, about 1/16" or 2mm
11) put the gears back on and check the alignment, mark the holes if any adjustments to the positions are needed.
12) drill the holes the size of the shafts you need for the gears. For the motor, drill a hole big enough to fit the gear on the motor. the location isn't critical for the motor, because its bolted to the gearboxs side, you can move it around and adjust it.
13) drill holes on each corner for bolts to run through it and hold it apart (togethor)
14) unbolt the two plates
15) place the shafts through a side plate and put the gears on, put the top plate over that
16) put prethreaded rod through the corner holes and use nuts to space the two plates the right distance apart.
17) mark the rods for length based on the width of the gearbox
18) make and attach plates to screw against the main plates over the shaft ends and retain them.
19) set the motor against the plate and mark where the screw holes. take it all apart, drill the holes, and re-assemble.
20) The last shaft in the gear train should go through the box. drill a hole through the shaft and put a pin through it that sticks out both sides. cut a slot in the last gear that fits the pin.