To make a dd or copy installable Super Grub 2 USB-key, you need to download SuperGrub2 finnish image and copy the image to an USB using cp or dd commands.
1. Find the USB using command
2. Go into the folder where SG2legacy.img locates and open Terminal and copy the image to the USB. An example if the USB is /dev/sdc:
sudo cp SG2legacy.img /dev/sdc
If using dd command, it goes like
sudo dd if=SG2legacy.img of=/dev/sdc
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.