Locate the Hobbed Drive Roller.

Slide the hobbed drive roller onto the stepper motor shaft. Note the orientation of the hobbed drive roller shown in the second image.

Press the red release lever and slide the hobbed drive roller down into the EZR Struder assembly until the midpoint of the hobbed portion is aligned with the split between the base and cover pieces (indicated by a green arrow)

Rotate the stepper motor shaft so the flat is in the orientation indicated by the blue arrow.

Align the hobbed drive roller so that the set screw will be set against the flat in the stepper motor shaft.