Lifting a corner of a car that weighs a few thousand pounds poses some challenges, but it's definitely possible.The first order of business in vehicle maintenance should always be safety. All passengers should get out and stand at least 10 feet (3 meters) away from the car and any passing traffic -- even if it's bitter cold outside, a torrential downpour or blazingly hot. Never, ever crawl underneath a car that's supported with only a jack. While a jack should do its job, it's not very stable and one bump can send the car crashing to the ground. If a car begins to move while it's on a jack, clear the area and wait for it to fall. Don't even attempt to stop it. Cars can be repaired, people are far more fragile.
Your owner's manual should show the proper placement for the jack. On unibody cars this is usually the metal seam that runs along the lower edge of the car on each side between the front and rear tires. This area is sometimes covered by a plastic fascia. However, there are generally openings along the fascia where the seam is uncovered. Placing the jack here will avoid damaging the plastic bodywork when the car is lifted. Cars with frame members typically have specific lift points, too. These are often indicated by a triangle cut into the frame metal, usually just in front of the rear tire and slightly behind the front tire.
Raise the jack until it's supporting, but not lifting the car. The jack should be firmly in place against the lift point of the vehicle and on firm, level ground. Make sure that it's positioned properly.
Remove the hub cap and loosen the lug nuts by turning them counterclockwise; however, don't take them all the way off yet. Just break the resistance. You can use the lug wrench that came with your car or a standard cross wrench will work, too. Your wrench may have different size openings on different ends. Be sure to place the right size wrench opening on the lug nut. The right size is the one that slips easily over the nut but doesn't slip or rattle.
Leaving the wheel and rim on the ground at this point means the force you're exerting against the wheel lug nut goes directly to the nut. If the wheel were raised off the ground then all you would succeed in doing is turning the wheel.
Remember, it can take a lot of force to break the lug nuts free. If you can't seem to free them you can use your body weight to push against the wrench handle, or you might even want to stomp on the wrench to help the effort. Just be sure you're turning in the right direction. Some drivers also carry a long pipe that fits over the wrench handle, adding even more mechanical advantage to the driver's efforts.
Once all the lug nuts are loose, pump or crank the jack to lift the flat tire off the ground. You need to lift it high enough to remove the flat tire and to put the spare tire on as well. Lift the car slowly while checking to ensure the car is stable.
If you notice any instability, lower the jack and fix the problem before fully lifting the car. If you notice the jack lifting at an angle or leaning, lower the jack and move it so that it can lift straight up.
With the tire completely off the ground, remove the lug nuts the rest of the way and then gently remove the flat tire.