How to set Plymouth Splash Screen?

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http://askubuntu.com – I have made a plymouth splash screen (its simple- it just displays a .png image) consisting of simple.plymouth, simple.script, simple.grub and wallpaper.png. Only, I cannot set it as default y editing the default.plymouth file (that errors) or through sudo update-alternatives --set default.plymouth /lib/plymouth/themes/MYTHEME/simple.plymouth it errors back at me saying no setting exists. How do you set this thing? (HowTos)