In general you connect the positive and negative of the source to the positive and negative of the load, but in your case we don't know what the source or load are.
If the laptop is old, and battery (in laptop) is also forever dead, and was rated 12V, you could always disassemble the laptop and connecting ground ang positive 12V from the car directly on the laptops battery contacts. After the battery is removed of course. But this is risky buisiness, so god luck
If you just want to charge the laptop battery using the car battery, then there are plenty of commercial alternators available for producing mains voltage for use in the car. Most of them simply have power sockets on them which you can plug anything into that would normally run on mains power.