Firstly we have made a modular mobile robot.
A detailed instructions are availalble: https://hackaday.io/project/12533-making-a-modular-differential-drive-robot
The CAD files for making the robot are available at GrabCAD : https://grabcad.com/library/modular-differential-drive-robot-1

For beginners, have a look at this video to assemble the robot: