It’s easy, not much Linux involved.
Set SD Card to enable USB boot, no need to prepare the USB drive, this is done with the imaging later:
https://www.raspberrypi.org/documentation/hardware/raspberrypi/bootmodes/msd.md
Then i used win32diskimager to create an image of the sd card and wrote that image to my usb hdd. (Etcher works similar, i guess.)