There are a total of 10 barn finds that will randomly become available as you make progress through the game.
The rewards are as follows:
1. 1965 Alfa Romeo Giulia TZ2
2. 1984 Ferrari GTO
3. 1962 Ferrari 250 GT Berlinetta Lusso
4. 1973 Ford Capri RS3100
5. 1954 Jaguar XK120 SE
6. 1945 Jeep Willys MB
7. 1967 Lamborghini Miura P400
8. 1961 Maserati Tipo 61 Birdcage
9. 1973 Renault Alpine A110 1600S
10. 1963 Volkswagen Type 2 De Luxe
As one becomes available, a large white circle shows up on your map and you must locate the barn. Once that happens, the car is picked up, and a little later you will be able to pick up the repaired car from your garage. After picking up all 10 repaired cars, you will gain the achievement.
View the video below for exact barn locations: