Select image (.ISO/IMG) you have already download from official HydraPWK pages.
Next step you need choose media make sure you have already insert your media.
Now you can use bootable media.
hydrapwk@hydrapwk:~# fdisk -l
...
Disk /dev/sda: 57,62 GiB, 61872793600 bytes, 120845300 sectors
Disk model: DataTraveler 3.0
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
...
My USB Flash Drive on ‘/dev/sda’ .
hydrapwk@hydrapwk:~# dd if=hydrapwk-amd64.iso of=/dev/sda bs=4M status=proggress
...