If you have the one with the vinyl portion, there is a zipper at the top you have to undo to release a piece of vinyl that goes behind the tire.
That piece is vital to keeping the cover on the tire, when I got my RAV it was folded under the lip (incorrectly) and the cover popped off in the carwash.
But there is nothing wrong with leaving the cover off, I have never put the new cover on that the carwash bought me...looks a little more rugged that way 8)
Worth noting that I have steel wheels, not many people going to steal those, which having no cover makes a little easier...
~Steve in Minnesota
08 Base V6 AWD Silver