1. Boot to the LiveCD Desktop

2. Open a terminal

3. Determine the partition with the Ubuntu/Mint installation.
sudo fdisk -l
If the user isn't sure of the partition, look for one of the appropriate size or formatting.

4. Mount the partition containing the Ubuntu/Mint installation.
sudo mount /dev/sdXY /mnt

Example: sudo mount /dev/sda1

5. Run the grub-install command as described below.
sudo grub-install --root-directory=/mnt/ /dev/sdX

Example: sudo grub-install --root-directory=/mnt/ /dev/sda

6. Reboot

7. Refresh the GRUB menu with
sudo update-grub