To change a flat tire, first ensure your vehicle is on a flat surface and turn on the hazard lights. Use a jack to lift the car off the ground, then remove the lug nuts with a wrench. Take off the flat tire and replace it with the spare tire, aligning the holes with the wheel. After placing the spare tire on, hand-tighten the lug nuts before lowering the car back to the ground. Once the car is stable, use the wrench to fully tighten the lug nuts in a crisscross pattern. Finally, store the flat tire and tools back in the vehicle.