Mobile robots are ubiquitous nowadays, and some companies build and sell them; but an inconvenience with these already built platforms is, for most of them, the limited access to their low level. Moreover, in spite of their ubiquity, there exists scarce information on how to build a complete mobile platform from scratch. This paper shows the detailed steps to build an omnidirectional robot based on mecanum wheels, for applications on autonomous exploration and navigation. The advantage of the holonomic configuration of the robot is the unconstrained instantaneous direction of motion. We provide a bottom-up guide for the construction of this robot, including the electronic devices, wiring, and the connection setup. Control and software frameworks are also detailed, and the result of the final robot together with some technical characteristics are presented.