On the opposite side of where the flat is, block the tire with a rock or other large item (the goal here is to keep the car from moving when it is jacked up).
Loosen, but do not remove the lug nuts. Do this before you jack the car up.
Jack the car up. Remember to get it up high enough to put on the spare tire.
Remove the lug nuts, removing the bottom nut last. While removing this last nut, keep you foot against the bottom of the tire.
Remove the flat tire.
Place the spare tire on the wheel. Push your foot up against the bottom of the tire to keep it in place.
Replace the lug nuts, starting at the bottom.
Tighten the nuts in a criss-cross pattern.
Lower the car off of the jack.
Do a once-over final snug of the lug nuts.
Be sure to pick up all the tools and flat tire.
Go to the tire store.
