Sort of OT, but I'm sure someone here will know.

I would like to visit some friends of mine who live in upstate New York. Previously, I've always flown to JFK, and taken Jet Blue up to Rochester. This time however, I'm thinking of flying to YYZ, and then driving across the border.

I'm a UK born, British passport holder, so have always entered the US on the Visa Waiver programme.

If I were to fly to Canada on BA, and then cross the border into the US by car how exactly would it work? Would I fill in the I94-W at the US border crossing point? Or would I have to apply for a full Visa before I left (spending a day at Grosvenor Square)?