I had mine delivered a week ago.
1) Prep flash drive first. Fat 32 like you said.
2) Ensure you
unzip the BIOS file (very important) from the download and put it into the flash drive (not compressed)
3) rename the BIOS file to MSI.ROM
4) I connected the motherboard 12 pin and the cpu 8 pin power (you might not need the cpu one), i did not have a CPU installed or the GPU, just PSU and motherboard power connector.
5) Ensure flash drive is in the USB right by the flashback button, i put it in the bottom one
5) Pressed Bio flashback button, it should then go into flashing the BIOS, you can see if its working by a flashing LED either on the flash drive or at the back of the circuit where the flash drive is plugged into as there is a white led back there, give it around 30 seconds to see if it kicks in.
It took about 3-5 minutes to flash the BIOS/
I followed this link and it worked for me for the 5600x, i'd recommend the BIOS under compatability in the MSI support download center for the MSI Tomahawk X570
https://www.youtube.com/watch?v=iTkXunUAriE
Hope this helps
W