After inserting your USB stick, you can find out what device it is using the following command:
grep -Ff <(hwinfo --disk --short) <(hwinfo --usb --short)
Finally, once you've found your block device, write the image to it. Point 'dd' to the full path such as '/home/user/Downloads/Linux-x86_64.iso'
dd if=/path/to/downloaded.iso of=/dev/sdX