Purchase the items in the component list above, then follow the directions here to setup the Pi: https://learn.adafruit.com/networked-thermal-printer-using-cups-and-raspberry-pi

Hold the Assembled Pi against the case and cut out a small slot in the side to make room for the USB->Wifi dongle to poke through, as shown in the pictures. Then drill a hole to match the input jack anywhere there is room in the box, and route the power wires back to your board.