Fisrt, use the tapping screw to fix the servo to the side board.(as below pictures)
Then, use the screw to fix the wheel to the servo.(as below pictures)
Use the rivet to install the castor wheel on the chassis.
Assemble the front board, the back board and the base board to the side board as below picture.
Use the binding post to fix all installed components.
Use the screw to fix the ring:bit to the micro:bit board.
Connecting wire as below pictures.
Fix the assembled ring:bit to the front board.