Here is the link to the Main board assembly. I will use this step by step and my own notes

Main Board web info

Here is a picture log (un commented)

1. Building step by step images here (uncommented). https://photos.app.goo.gl/nMbYL65gHW5JvAyB6
2. Soldering skills needed and need a nice tip. I use one mainly a curved one for my Hakko FX888D. I am running 710 degrees at the tip. On the ground connections a significant amount of heat is needed to get the solder to flow. Heres the tip. https://amzn.to/408oD3n
3. You will need to solder both headers and pin sections. Take your time as this is not a race. Note, once soldered on this 4 layer board moving things about or removing the pins/header will be impossible with a standard solder sucker. One must needs a Soldering Desolder like the Hakko FR-301 (it is expensive). If you are careful, you will not need to remove anything.
4. As you assemble, at the power section. The heatsinks are ISOLATED from ground. This thew me off a bit as the 7805 5v tab is indeed gnd and connects to the center pin. However the 3.3 Volt regulator the LM1117T 3.3 the tab is the OUTPUT. (this threw me). As I had higher current readings. (not the cause see below).
5. After you have added all the parts per the instructions, and verified each and every connection measure the 12V Vin resistance with your meter. At the 12V input ~ 20 megohms. At the 12V output 10 megohms.. leave the meter there to measure.. it takes a bit for readings to stabilize.
6. At TP1 5V ~3.7Kohm takes a while to stabilize as there are large 470uf caps at the output
7. At TP2 3.3v ~560ohms takes a while to stabilize as there are large 470uf caps at the output
8. Assembly went very well, note no power has been applied yet.
9. At first, the Author will have you apply power to the 12 OUTPUT connection while looking at the current. It says it bypasses the Power on circuitry big Mosfet U4 SUP90P06. Well, no.. Upon power up I found that getting a full 12 at the input with my Current limited supply I needed 100ma (not 25ma). If one removes U4 you will get ~25ma.. and the right voltages. Well, after several hours of scratching my head.. 100ma should not be a problem and I verified the voltages 5, 3.3 and the 5v (yet another regulator U2 for the ATTiny only. It was 5v from Vcc pin 8 to ground pin4). I figured out that the power on circuitry upon power up puts 12v at the gate of U4.. droppin a lot of current across it.
10. I then did a test, the voltages for the ATTINY were fine. So I attached the pushbutton switch to the white connector at the front of the AtTiny (Notch towards center of board). and inserted the tiny.
11. I monitored the gate and the current at my supply. Upon power up, the gate was high at 12V and the current was ~100ma. I then pressed the button or held the 2 wires together for a short time. The Gate was clocked and went low.

The current dropped to 25ma. and voltages were fine! and the current was down where it needed to be.

1. I then shut off the power and switched the input voltage to the 12V Vin. Nothing at 12V VOUT. Powered back up. NO CURRENT was being drawn and the GATE of U4 was high. Ok.. so I pressed the switch or shorted the 2 wires for a second.. the gate of U4 went low, power to the regulators came up and the current was ~25-30ma.. perfect.
2. I then powered down, attached just the Teensy and powered back up. Again, no current drawn, (there is 5v on the Teesy btw). Pressed the button or shorted the wires for second.. the unit came up. Teensy orange led, and power supplies all up..
3. I then could turn the unit on AND off by pressing the button or shorting the wires. It takes a couple of seconds to shut down but it does..(the save routine in the teensy). Current Draw with the Teensy and ALL is 185ma (fluctuates as it comes up and powers down). This is ~20-30 ma greater than what AI6YM measured.. but it is stable, nothing getting hot or warm.
4. Ill call it done for now. Neat little shut down startup routine based on charging and discharging a cap to clock the teensy.. I would have never thought of that. (I am not sure of how many charge/discharge pulses are needed to reset the ATTiny.. If one does it quickly.. there is not enough pulses and the unit will not shut down. Starting it up is quick.. short press.. longer press to shutdown.. I think.