If you have access to a linux machine, the easiest way is to follow the directions at the 10:30 mark of the video. You basically get a beta branch of the python code and you do not need to push the button to flash: JelmerT/cc2538-bsl at feature/ITead_Sonoff_Zigbee-delay (github.com)
That is the only way I have done it. I may work on windows too, not sure.