Solder everything together. SMD soldering isn't hard if you are afraid of that. Using solder paste and hot air will probably be faster than a regular soldering iron.
Everything fits so it is time to create the shell. Cut the sides and glue them in a 90 degree angle to the top plate. Finish it by sanding the edges and some paint.
Acquire parts and follow the hookup guide in the files above for radio assembly. Then follow the project log to replicate what we have done to build everything.
Perform cable management, insulate the Arduino, glue it into place with hot glue (make USB accessible for further software changes). Make sure everything works, close your balance scooter and enjoy.