Find a heavy duty card stock or other thicker paper. You do not want it to be thin, otherwise it is going to fall over and not hold up if it is outside. You also want the paper do be a bright, vibrant color, such as neon yellow or bright orange. This way people can see it, even at night.
Write using your pencil in large letters "For Sale" at the top of the sign. It is a good idea to use a pencil before the marker. You don't want to accidentally spell something wrong or run out of room.
Print what is for sale under the "For Sale" Make this brief. If it is a car just write the car name and year. You don't need to write a paragraph description of the item you are selling. Just whet the people's appetite with what you're selling, don't give them the entire meal.
Write your phone number neatly. Do not give out your address just yet. If the person is really interested they can call and receive your address, or you can meet somewhere.
Trace over all of the information with the permanent marker. Allow it to dry and it is ready to be displayed anywhere from your car window to on a street sign.