I'll be there I think (last outing before Le Mans!)
It officially starts at 8 so I would have thought any time from 8-9 and you'd be fine. Although this one is the most popular one so might be worth getting there for 8.30ish.
You just turn up. Entry is free. If you come in a Supercar then the marshals on the gate may select you to go through and onto the grid or paddock. Normal tintops will be parked in a nearby parking field.