First, drive slowly to a stretch of flat, firm road that’s out of traffic. Pull the parking break, put on your hazards, and set out emergency triangles if you have them.
Next, remove the hubcap. (Follow your car manual.) Use a lug wrench to loosen the lug nuts. Give each a turn counterclockwise, but leave the nuts in place to keep the wheel attached.
Position the jack. (Consult the manual.) Turn or pump its handle until the tire is four inches off the road (never, ever put any part of your body under the car). Remove the lug nuts; pull off the tire.
Slide on the spare. Screw on the nuts (working in a star pattern, not clockwise) and tighten. Lower the car so the tire just touches the ground. Fully tighten the lug nuts.
Finally, lower the car fully. Remove the jack. Give the nuts one last turn with the wrench (use muscle!), then get to a mechanic.