Glue the DC motor into hole you cut earlier.
Feel free to use a lot of hot glue here, this motor will be spinning fast and we don't want it to vibrate itself loose.
Check that the wheel lines up with the hole cut in the top of the platform and doesn't rub against the edges at any point.
If you haven't already fixed the position of your piston, now is the time. Position servo motor on platform so that the piston is just at the edge of the opening in the tube when the rotor is all the way back.
Use double sided tape to stick the entire magazine + piston assembly onto the launch platform, aligning it in front of the spinning launch wheel.
Everyone has a different way of managing the cables and wires that come with any electronics project. In this case, it helps to position the battery pack and Circuit Playground Express board on opposites sided of the handle, and use zip ties to keep messy wires held together in tidy(ish) bunches.