So there you are, driving along the Highway 1, music playing from the stereo, your boyfriend/girlfriend wind blowing in your 7 dollar haircut when BAM, you get a flat tire. You pull over to the side of the highway and things go steadily downhill from there; the stereo announces rain coming your way, your boyfriend/girlfriend starts to complain, and your 7 dollar haircut loses it's charm without the wind, leaving you looking somewhat like Cameron Diaz in There's Something About Mary.. you know what I mean.

Anyhow, the pressure is on. What do you do? Simple. Pull out your trusty laptop and cellular modem. Connect and log into Everything2.com and start following instructions.

First of all, take out the tools you will need. First you need, and I can't stress this enough, you NEED a spare tire. What, you don't have one? Then take your laptop again and play a game of Quake because quite frankly, you're beyond help. who the heck travels without a spare?? Ok, now the rest of you who have a spare, (glare at fool with no spare), next you need a tire iron and a jack, notice the lower case letter, not Jack as in your boyfriend/girlfriend. Also insure that your car is on a flat surface so as to prevent it from slipping and landing on a surface that resembles say, your body.

Step 1:
Remove the hubcap from the tire(the flat one!) using the tire iron to pry it off. Place the jack underneath the car. Ensure that as you jack up the car, the pressure is place on the car's frame or body, and not, for example, on the gas tank. Raise until the flat tire is a couple of inches off of the ground.

Step 2:
Proceed to unscrew the bolts from the tire using, you guessed it, the tire iron! The bolts will generally be very tight and will require a lot of pressure to unscrew. If you're not really sure which direction to turn in, i recommend throwing the tire iron away and jumping to the left(or right if you're on the other side of the highway) into traffic. As you remove the bolts, place them into your hubcap so as not to lose them. I need to mention this because as we have seen, some people cannot even remember to bring along a spare tire.

Step 3:
Remove the flat tire and replace it with the spare. Screw in all of the bolts(never drive with less than 3, of course if you lose any I don't recommend driving at all.. ever), replace the hubcap and lower the car back to the ground. Place the flat and your tools back in the trunk and drive away, once again allowing your 7 dollar haircut to shine in the wind.

Notes: Although presented humorously, there is nothing funny about a 7 dollar haircut.. well, unless it isn't on your head and you have the oppotunity to laugh at the person.

Oh yes, and I was serious; this really IS the proper way to change a flat tire.

Very well said, but I would add one recommendation:

Do not jack the tire up a couple of inches prior to loosening the nuts. It's much easier to loosen your nuts if you leave the tire on the ground and then jack it up.

Great writeup save for one slight detail:

You will want to put the nuts on with the beveled side inwards, so they push into the metal of the spare. Otherwise, there will be a slight amount of space regardless of how tight it's screwed on and the wobbeling of the road will loosen the nuts. At the extreme, this can cause the tire to come off and your car to spin, potentially messing up your seven dollar haircut quite severely.

It's also important to remember to have the handbrake firmly pulled before hoisting the wheel off the ground.
If you have a 'fake' bolt to guide the spare tyre to the right position this should be screwed in where the bolt that's highest is BEFORE you remove all the other bolts.
...and last but not least the bolts should be tightened diagonally in order to make the wheel sit as straight as possible.
Oh, and if you didn't know how to change a tire you probably also didn't know that the wheels have to be balanced individually for maximum grip, fuel economy and what have you.
Therefore you'll need to get the old tire fixed and put back on as soon as possible.