The STL file can be found in the files section. The ring size is 8, so it may not fit everyone's pointer finger.

Wire the "+" and "-" terminals out. It's recommended to use 30-32AWG enameled copper wire.

In the picture above and all remaining instructional pictures, the red wire signals power or "+".

Connect the red wire (the battery's "+") to one end of the switch and solder another wire to the other end.

Slide the XIAO board into the ring base, using hot glue or super glue to secure its position. Connect the battery's "-" wire to the XIAO battery "-" connector, and the other wire on the slide switch to XIAO battery's "+" connector.

Test the circuit. The XIAO should now be able to be powered on and off by the switch (the power LED on XIAO board should light up).

Before moving on, you can read about this board and its pinout and uses here: https://wiki.seeedstudio.com/XIAO_BLE/

The TTP223 touch sensor board is too big for the ring, but we can cut out a small section of the board with only the chip to make it work. This can be done with pliers and doesn't have to be perfect.

If you are unfamiliar with this board, here are some wonderful instructions for it: https://www.instructables.com/Tutorial-for-TTP223-Touch-Sensor-Module-Capacitive/

Once cut, solder wires to the board as indicated above with for wires: power, ground, output, and extending the touch range. 

Be careful with the TTP223 boards—both with cutting and purchasing them. There are two versions of the board: one version resets the output after approximately 8 seconds (meaning that you can only activate the sensor for ~8 seconds continuously before it automatically deactivates and you have to move your finger to reactivate it), while the other allows the sensor to be continuously activated with no limit.  For ErgO, it is preferable to get the version with no timeout, but it's very unclear which version you are buying and it's rare for sellers on Amazon and AliExpress to explain which version they have. Test the sensors after you buy them. If you bought the ones with a 8 second time limit, follow the next instruction 6 on how to change the chip out or buy new ones. If you bought the ones without a time limit, skip ahead to step 7 and directly mount the boards after cutting.

Only do this step if you have the TTP223 touch sensor with a 8 second activation time (read step 5 above for more details). A way to get around this time limit is by changing out the chip on the TTP223 with the AT42QT1011-TSHR (can be purchased and read about here: https://www.digikey.com/en/products/detail/microchip-technology/AT42QT1011-TSHR/2268868). Carefully remove the current chip and solder the AT42QT1011-TSHR on.

Secure one on the bottom left and the other at the top right using either hot glue or super glue (use very little). The orientation and exact positioning doesn't matter since we will be using the touch wire (reference the image in step 5) to extend the activation area. The wire conducts the board, so we don't have to touch the TTP223 board itself to activate it, we can instead touch the touch wire.

There are four wires need be connected as such for each touch sensor board:

VCC (for both) --> XIAO VCC (3V3) 

GND (for both) --> XIAO GND

Output (for the upper right board / keyboard activation) --> Xiao D10

Output (for the lower left board / mouse activation) --> Xiao D6

Don't get rid of the touch wire just yet, it will come into use once other components are attached.

Try on the ring and position the trackball in a way so that it's easily and comfortable reached by the thumb. Try to fit it snugly so that it's level on top of the XIAO microcontroller. DO NOT GLUE IT DOWN YET. We still need to wire some components to the Xiao, so don't glue the trackball on until the very end.

Wire the trackball as so:

VCC (3-5V) --> Xiao VCC

SDA --> Xiao SDA (D4)

SCL --> Xiao SCL (D5)

GND --> Xiao GND