This is the Corne v1-v3(rev1) firmware guide。
Use Remap
The easiest and most recommended way is to go to Remap and select FLASH for crkbd:via.
Once the firmware has been written to one side, follow the same procedure for the other side.
Use QMK Toolbox
Download the firmware of VIA from crkbd_rev1_via.hex
With the keyboard connected via USB,
press the reset button twice or short GND & RST pins on ProMicro to start flashing the firmware.
If you see the message, it's done.
Once the firmware has been written to one side, follow the same procedure for the other side.
Please refer to https://github.com/foostan/kbd_firmware.
VIA can be used by flashing the firmware for VIA.