How do I charge my lawn tractor battery with my car?

You don't charge with your car but its called jumping. Put jumper cables on your car terminals, red hot and black is ground. Put the red cable other end on your tractor battery and clamp the black cable to the engine block. This will give your tractor starter plenty of power to start. If you were to leave these cables connected for hours with the car running at higher than an idle spped you might actually bring the tractor battery charge up. I would spend $35 at Sears or an auto center and buy a small charger.