use lsblk to find where the sdcard or usb stick is mounted. Something like sda.
I’ve read other sources advising to unmount the boot image from /run or /media before writing to the flash drive, but I never had to do that.
Then use the dd command the write the image. image files can be .img, .raw or .iso depending on the architecture and the vendor
dd if=pathto.img of=/dev/sdb bs=4M iflag=fullblock oflag=direct conv=fsync status=progress