I used https://www.vandyke.com/products/securecrt/ for flashing...
In general these were good instructions...
https://sonoff.tech/wp-content/uploads/2022/11/SONOFF-Zigbee-3.0-USB-dongle-plus-firmware-flashing-.pdf
Only thing I would clarify
"Keep pressing the Boot button, restart the device..." to "Keep pressing the Boot button and press restart button at the same time."