Where are you from?
Actually it is called to ground, not to earth. I don't even know if everybody in here understood what you were saying...
(If it's okey to say earth in english/american, it is just my english/american which sucks)
Anyways, on to the question; To ground something, you connect it to the car chassis. Scrap of any paint/dirt from the point you deside to use, to ensure optimum contact.