Just make the dealer what you think a reasonable offer is. They'll either take it or counter offer. But then be careful especially if your trading in another vehicle. The dealings can get _very_ complicated and you usually come out on the more expensive end of the stick.
Make the deal on and out the door price, all taxes and fees etc. If at time of delivery they want to add a couple of hundred dollars for something they "forgot" its a lie, _RUN_ don't just walk away