This Blender 2.8 tutorial explains how to design a simple 2-stage gear train with coaxial input and output shafts and given gear ratio (1:12 in this example.) The resultant mechanism can be used as a toy clock with the minute hand attached to the input shaft and hour hand to the output shaft.
The tutorial uses a short Python script to compute the number of teeth for each of the gears involved. It also demonstrates how to turn a gear with a low number of teeth and profile shift to a solid object without creating overlapping faces.
The end result of this tutorial, a .blend file, can be downloaded via the link below.