Flat tire on the road, how to repair it temporarily?

First thing is to find a safe spot away from traffic, put out your emergency triangle, and turn on your hazard lights. If you have a temporary repair kit (those sprays or patches that come in a lot of cars), follow the instructions exactly - stick the tube in the valve, spray it good, and wait whatever time it says. Then inflate the tire with a portable compressor or head to a nearby gas station. That'll get you to a shop, but it's not a permanent fix - you need to actually repair or replace the tire as soon as you can.