To use the Router-CIM quick UCS commands, you simply need to enter the command (FS, RS. BS. LS) at the command prompt, and when prompted for the Origin location, select the Lower Left Corner of the part.
The normal World Coordinate system is set to the lower left corner of the top of the part:
To use the side UCS commands, you need to select the lower left corner of the face you are going to work on. For example, when using FS, you will be prompted:
fs Origin point <0,0,0>:
From this prompt, select the Lower Left corner of the front side of the part
To demonstrate, there is a drawing showing the correct point of each of the 4 sides.
The basic point to make is that after entering any of the 4 quick UCS commands, you must choose the Lower Left Corner of the face you are about to work on.