Do you know how to flash a firmware (esptool)? Did you the find the generic documentation about this firmware? Do you have specific questions left?
I understand how to flash. I'll probably use the Arduino IDE.
I must have skimmed over...
The “plain” image consists of two files: espiobridge-plain-iram-0x000000.bin and espiobridge-plain-irom-0x010000.bin. These can be flashed to address 0x000000 and address 0x010000 respectively, using your flash tool of choice.