You must install motors first, before installing an Arduino board. 

Bill of materials

• Assembled Decabot chassis with motors (Step or DC motors).
• Arduino board or similar in form factor.
• 3mm bolts and nuts (3 sets), and a screwdriver.

Instructions

Prepare the materials, the board, nuts and bolts.
Put a bolt through the marked hole from behind, and fix it with a nut.
Screw this nut through the hole marked with an arrow.
This first nut will work as a support for the Arduino board.
Position the Arduino board and put a bolt on the right top hole.
Position a bolt on the internal chassis part to fix the bolt.
Screw tight the bolt to fix the Arduino board.
Do the same with the right bottom arduino screw hole.
If you need access to your USB Arduino port, you must remove the USB cover from the shell.
This allows you to connect an USB cable for upload sketches to the Arduino board, or to power it up.