Generally a new boot cover will come with all the clips, snaps and butterfly strap elastics neccisary already installed on it.
(al the bits are part of the cover, not the car, with the exception of the other half of the snaps)
From there its as howndog said: they clip onto the trim around the edge of the body.
If you've no snaps present on the B pillars just behind the plastic trim, that can easily be reconciled once you have a cover to install and get things lined up as it's a standard male snap piece that gets retained by a self drilling/self tapping body screw like most all the others in that area.