If you get a flat tyre whilst you're about and about, try to find somewhere firm and level to park. Switch on your hazard warning lights, turn the engine off and apply the handbrake. If you're forced to change a wheel with your car on soft ground, you will need to place a plank or something similar under the jack to prevent it from sinking into the ground.
If you have a manual transmission, engage first gear if the car is facing uphill and reverse if it's facing downhill; if your car is an automatic, leave it in 'park'. Get all the passengers out of the car and safely away from the road. If you have any luggage in the boot, take it out if it's obstructing access to the spare wheel and tools. If you have an emergency warning triangle, you should place it about 30-50 m behind the car.