Find a place where movie filming will occur. When Forrest Gump was made, the people sitting on the park bench with Tom Hanks were extras from the local community. Also, an old college roommate of mine found a news article that a film was coming to town starring “Dr. McDreamy” (Patrick Dempsey) b4 he was famous in Greenville SC. They were told in the article that the crew were looking for extras to drive 1960s model cars and to dress appropriately. She and her mom went and were chosen to drive a car.
So, all you have to do is know where a movie is going to be filmed and show up for the extras casting call. Most easy role: person in the crowd. Generally films are made with settings in the “present”, which means all you do is dress normally to go to the casting call for extras. Those are usually found in the Entertainment section of your local newspaper. If you live near a major city, you can subscribe to that newspaper in order to know if a film crew is coming to a town near you.

Good luck!