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Safety first, always. Pull completely off the road onto a flat surface — a gentle slope can cause the car to roll off the jack. Turn on hazard lights, apply the parking brake, and if you have them, place wheel wedges or road flares behind the tires. On a freeway, call roadside assistance rather than attempting a tire change in a live lane.
Loosen the lug nuts before jacking up the car. This is critical — you need the ground contact to keep the wheel from spinning as you break the initial torque. Turn each nut counterclockwise to loosen about half a turn (don't remove yet). If the nuts are extremely tight, stand on the wrench handle or use a breaker bar.
Find the correct jack point — it's in your owner's manual and usually marked by a notch or reinforcement just behind the front wheels or in front of the rear wheels. Jacking at the wrong point bends the car's frame or plastic underbody panels. Never put any part of your body under a car supported only by the scissor jack.
Once the car is raised and the flat removed, put on the spare. Hand-tighten the lug nuts in a star pattern (opposite corners, not circular) to seat the wheel evenly. Lower the car until the tire touches the ground but doesn't carry full weight. Then fully tighten the lug nuts to spec — typically 80–100 ft-lbs — in the same star pattern.
Compact spare tires ("donuts") are temporary — maximum 50 mph and maximum 50–70 miles. They're not designed for highway speeds, cornering, or extended use. Get your flat repaired or replaced as soon as possible. Full-size spare tires (if your vehicle has one) can be driven normally until you reach a shop.