install grub boot-blocks

install

ubuntu

apt install grub-pc

slack150+

v15.0 now has the package available

slackpkg search grub-2

from source

see grub-source

requirements

define your target disk

fdisk -l
disk=/dev/sda
disk=/dev/nvme0n1
disk=/dev/loop0

define the boot folder within ?

# (default)
boot=/boot

# mounted from another system
boot=lala/boot

gpt

in case it is a GPT table, do this first (BIOS BOOT PARTITION)

cgdisk $disk

(about 1M free space is more than enough)
(it does not matter what partition number / slot you take)
type --> ef02
name --> bios-boot

partprobe $disk
fdisk -l $disk
ls -lF ${disk}*

ready to go

in any case, you can then proceed

grub-install --target=i386-pc --boot-directory=$boot $disk

grub setup

see grub