Problems and solutions.
Wires do not strip easily. I found no matter what I did the black wire came out of its insulation, so I had to use electrical tape to prevent any shorts. You may want to try a different type of wire. Soldering the wires red on top and black on the bottom. I kind of assumed that the black wires and red wires would stay together when I inserted them into the motor port of my DFRobot Dual Motor Controller RB-Dfr-19. This was not the case as I had one motor going forward and the other going backwards. To get them in sync. I had to have the black wire of one motor and the red of the other connected to either the + or - of the motor port.
Battery case. I finally chose not to put the battery case in the compartment with the motors. It was too difficult to get to for changing the batteries, plus I was afraid I would disconnect one of my motor wires. I took the small rectangular metal piece that might have been for sensors and mounted it to the front of my robot platform. I had to drill two different holes to mount it, but it is much easier to get to the batteries. I could have easily made this the back of the robot and put the switch under it so that it could not be knocked on or off accidentally. Now with my battery case were I can get to it and my motor Controller mounted on the small standoffs, just above the motors I can do all my prototyping on the top of the robot platform.