How do Skins (Turtle shop) work?

[Merikkinon]

So, how exactly do they work as far as applying them?

Do I get to still choose hairstyles and all that bling stuff?

Is this like a one-time change and can't be changed back? Or is there an item I use to actuate it and I have access to that item (like the glyphs for Turtle, HC, Twink)?

Just curious.