sudo fdisk -l

### Mounting ###

sudo mount /dev/sda# /mnt            #Mount root (/) partition
sudo mount /dev/sda# /mnt/boot       #Mount boot (/boot) partition 
                                      (if separate from root partition)
sudo mkdir -p /mnt/boot/efi          #Create EFI partition mount point
sudo mount /dev/sda1 /mnt/boot/efi   #Mount EFI partition

sudo mount --bind /dev /mnt/dev
sudo mount --bind /proc /mnt/proc
sudo mount --bind /sys /mnt/sys

sudo chroot /mnt                     #Chroot to your installation

### Installing ###

sed -i '/cdrom/d' /etc/apt/source.list

apt-get install grub-efi-amd64  #Install grub EFI bootloader

grub-install --recheck --no-floppy --force
                                     #Install grub bootloader in EFI partition

echo "configfile (hd0,gpt#)/boot/grub.cfg" > /boot/efi/ubuntu/grub.cfg
                                     #Tell grub to load grub.cfg from /boot

update-grub                          #Create grub menu list

exit                                 #Exit chroot

### Unmounting ###

sudo umount /mnt/dev
sudo umount /mnt/proc
sudo umount /mnt/sys
sudo umount /mnt/boot/efi
sudo umount /mnt/boot
sudo umount /mnt