1. Check disks and partitions
2. Unmount USB’s active partition.
sudo umount /dev/sdXY ## where X=disk and Y=partition
3. Open Terminal and go to the folder where the image is or open Terminal in that folder.
4. Copy image to the USB
sudo cp grub4dosBOOT.img /dev/sdX ## where X=disk
Warning! If you are not sure what you are doing, do NOT do!
Edit: Safer way to put the image to an USB is to use Etcher.
About testing the Grub4Dos boot-USB.