There is this documentation https://github.com/radxa/documentation/tree/master/rs102 that really should go on the wiki.
Its fairly easy really and you don’t need a serial console but does give you some feed back that the Zero is booting in USB mode.
Its pretty easy as ‘lsusb’ will not find it on the host computer so as said not essential.
So you can jump all the serial console and just
sudo pip3 install pyamlboot
sudo apt-get install fastboot
You prob need a linux machine and ‘sudo apt-get install python3-pip’ or use a live image off a usb stick.
You could also install python on a Windows machine but can not say how that will go but likely will work.
But by keeping the little USB boot button pressed on the back of the board when you apply power the bootloader almost instantly switches to usb mode.
You just download by, or copy and paste the url
wget https://dl.radxa.com/zero/images/loader/rz-fastboot-loader.bin
Then the command is
sudo boot-g12.py rz-fastboot-loader.bin
You can follow the top document and just wipe anything off the eMMC.
To get the Zero to load up as if its a USB stick its just a different download after a power cycle.
wget https://dl.radxa.com/zero/images/loader/rz-udisk-loader.bin
Boot into usb mode and this time
sudo boot-g12.py rz-udisk-loader.bin
Then you can flash your image just as if it was a usb stick or sd card in a reader.
Last thing
wget https://dl.radxa.com/zero/images/loader/u-boot.bin
sudo dd if=u-boot.bin of=/dev/sdx bs=512 seek=1
/dev/sdx x is whatever driver letter linux applies.
lsblk
Will list unmounted drives