arch-chroot /mnt packman -S grub-bios
  or grub-efi

arch-chroot /mnt packman -S os-prober
  to set up the package os-prober for detecting other OS-es

genfstab -U -p /mnt >> /mnt/etc/fstab
  -p - print, /mnt - for this folder
  additional options
    -U - UID (not changes while cloning fs-ses to other destinations)
    -L - for labels

arch-chroot /mnt
  use the current kernell of installer, but change the root 
  !!! opens a new interpreter (sub-shell)
      run the bash for better comfort from there