Follow your favorite method for the initial Ubuntu install.

Once Ubuntu ins installed, make sure the command grub-customizer is available on your system (it is available from Ubuntu repositories).

Create 2 consecutive partitions on your hard drive

• partition 1 : type = fat32, name = BATOCERA
• partition 2 : type = ext4. It must be created right after the BATOCERA partition.
• download boot.tar.xz and unzip it on the BATOCERA partition.

Use grub-customizer: create new and write BATOCERA as its name or anything you like, with type “other”.

Then, with gparted, find the UUID from your BATOCERA partition. In the example below, 3377-44C4 is my UUID, don't forget to adapt to yours.

Paste the following code:

insmod fat
search --no-floppy --fs-uuid --set=root 3377-44C4
linux /boot/linux label=BATOCERA console=tty3 quiet  loglevel=0 vt.global_cursor_default=0
initrd /boot/initrd.gz

Save and reboot.