Copy the files to /usr/lib/systemd/system/ and run:

systemctl enable bootsplash-ask-password-console.path
systemctl enable bootsplash-ask-password-console.service
systemctl enable bootsplash-hide-when-booted.service
systemctl enable bootsplash-show-on-shutdown.service
systemctl mask systemd-ask-password-console.path

Edit the /etc/mkinitcpio.conf, find the HOOKS= line and add: bootsplash-XXXX (XXXX is the name of the theme you have chosen)

Remove the quiet boot parameter and add bootsplash.bootfile=bootsplash-themes/XXXX/bootsplash (XXXX is the name of the theme you have chosen). For GRUB users, this can be done by editing the GRUB_CMDLINE_LINUX_DEFAULT= line in /etc/default/grub

Regenerate the initramfs : sudo mkinitcpio -p linux

bootsplash-hide-when-booted.service is only needed for text mode use, as otherwise the splash won't go away when the console is ready. If you only ever boot to X, then you can leave it out.