The PCB is mounted to the top enclosure part with a single screw. Fasten the screw all the way until it's flush with the enclosure.
Place the slide switch into the cut out with two clips. Wrap the wiring around the socket header and through the GPIO pins. Insert the slide switch into place.
Plug in the JST connector from the 2500mAh battery into the JST port on the Powerboost 500C.
Place the battery on top of the 3.5" PiTFT away from the GPIO socket and header.
The 2500mAh lipo battery can rest over the wiring. The JST cable is behind the slide switch in this case. The battery should stay in place once the Raspberry Pi A+ is plugged in.