Use a flush cutter to carefully cut the tabs off the Dotstar matrices. This doesn't have to be a clean cut -- you should intentionally leave a lot of material on the grid so that you can sand it to press-fit into the frame

This is approximately how much material should be left from the tabs on the edge of the grid before sanding

Carefully use some sand paper or sanding stick to smooth out these rough edges, but continue to check if the grid will press-fit into the 3d printed frame. You want to keep checking the fit as you sand to avoid over-sanding. 

It bears repeating: it is BETTER to sand TOO LITTLE, than too much.

Take your time with this step -- the frame will very slightly expand as the LEDs get warmer, so you want to err towards the grid needing a little bit more force to press fit in.

You'll want to make sure that the switch cutout is oriented at the "top" position, and then orient each piece as shown using the back of the grid as a guide (this will result in the cut tabs being on the north and south of each grid)

Again, don't sand too much. When in doubt, just move on to the next steps, you can always sand some more in the final assembly steps.

Desolder the JST connector AND the power switch on the QT Py charger add-on, making sure that the power pads are left intact. The photo here shows what the charger add-on will look like before and after desoldering.

The red arrows are the two pads that we'll need to connect to turn on the circuit, and the pads marked (-) and (+) denoted by the red square are the pads we'll need to connect the battery wires directly to.

Solder two wires to the tiny power switch, one onto the middle pole and another on either side poles.

It will help if you tin both the switch poles and the wires with some solder before you solder them together.

Solder each wire from the switch onto the on/off pads on the charger add-on. This switch will now act as the power switch for the entire circuit.

Insert 7 pin headers into some breadboard, and align the right side of the QT Py onto these headers (the side with the 5V and GND pins). Then, insert a single pin header into the breadboard on the other side of the QT Py, and align the TX pin of the QT Py onto this single pin. Refer to the photos to see this a bit more clearly. There should be seven pins on one side, and one pin on the other side.

Solder these headers onto the QT Py!

Note that you could also choose to fully solder 7 pins on each side of the QT Py, it might just be a bit more challenging to solder the wires connecting to the LED grid later on.

Now that you have all the headers soldered onto the QT Py, you can now align the BFF charger add-on to the back of the QT Py. Ensure that the 5V pin is aligned with the 5V pin of the QT Py on the other side. 

Cut off all excess headers as short as you can with a flush cutter.

Using your trusty flush cutters, cut off one wire as close to the connector as you can to get the max amount of wire length. Then, cut the other wire off. Do NOT cut both wires at the same time.

Use a wire stripper to expose the wires, and use some solder to tin each wire. Then, with the help of a third hand, carefully solder the red wire to the positive (+) pad and the black wire to the negative (-) pad. 

Now, you should have an assembly of the QT Py, the charger add on, the battery and the switch which should look like the photo here. 

Try to turn on the switch. You should see the LED on the QT Py light up if you haven't modified the factory code onboard.

Great job! Admire your work for a second before moving on