[Mr.ac]

Preparing is nothing more than making sure the parts are clean and contaminate free.
Meaning just like in painting you don't want to have dust on the surface.

Simple as that.

As for wrapping an unpainted bumper, I don't know. If the primer I smooth enough and the wrap is thick enough it could be done. Unless it turns out like orange peel surface.

But that is something I would talk to a pro about.