If you have a one-star car, you will be able to convert the one-star Wild Card (of the same class as the car) for that car.

Once your car is starred up to two stars, you can start using two-star Wild Cards to keep upgrading the car.

When you have enough Wild Cards to star up a car, it will be highlighted in green in the blueprints bar. Tap the icon to apply them. You can also exchange Wild Cards for Trade Coins.