In a bowl, combine sour cream and taco seasoning until blended. Cover and refrigerate for at least 1 hour.
To prepare tostadas, top each tostada shell with a spoonful of beans and spread evenly over the shell. Then spread with a spoonful of guacamole followed by a spoonful of sour cream spread evenly. Then sprinkle with some of the cheese, tomatoes, olives and green onion. Makes 12 tostadas.