I am assuming that you have an adjustable buck converter (you can get USB converters that are probably more convenient for this task - they take in various voltages and output 5v through a USB connector like this one: https://www.amazon.com/dp/B01NALDSJ0/ref=cm_sw_r_cp_apa_glt_fabc_N7HA7X041TRP40BHMVF1)

I needed a variable output to drive my 12v LED lights, so I used this one: https://www.amazon.com/dp/B08CRSL41Q/ref=cm_sw_r_cp_apa_glt_fabc_APM6Y1CTYVH9EHRSNPXZ?_encoding=UTF8&psc=1)

I soldered a 500 ohm resistor across the output pins. This will constantly provide a load to the converter. I'm not certain why it is designed this way. You will need a voltmeter and a small screwdriver to adjust the output voltage. I would not connect the pi until you get the voltage close to 5v.

Don't forget to cut off the power and ground wires to the pi when connecting the USB cable to the printer or you may blow the printer and/or the pi.

It's really not hard. If you just want to power the pi, get the USB converter, connect it and you are done.

If you have the adjustable one, connect the resistor, adjust the voltage, and then you can connect the pi.

MVSX runs off 12v(DC) if I recall, MiSTer runs off 5v(DC). You would need a buck step down converter to drop the 12v to 5v (need at least 2amp).

Don’t cheap out, get a good one and test it before use.

Last time I did something like this I used these (unavailable): https://www.amazon.com/gp/product/B01NALDSJ0/