Instructions
Load your game profile and select a race to play. The easiest way to get stars is to go back and play races in which you have already completed but didn't get first, second or third place. You get three stars for first, two for second and one for third. Go back and get a higher place on some of the easier tracks.
Press the "L1" button before beginning a race while in the loading screen to see a list of stars that you can earn by performing specific tasks in that race. Some of these stars include beating a top lap time or completing the race in a certain place. Each race will have a few of these that you can complete for easy stars.
Interact with other cars on the racetrack during any race to earn easy stars. Get directly behind one of the other cars and follow it as close as you can. Not only will you get a speed boost by riding its tail, but you will also earn stars for the maneuver, which is called "drafting." Get to the side of the other cars and swerve into them. This move is called "trading paint," and will also earn you stars.
Wait for the yellow light to appear on the starting line of any race. Press the accelerate button and move the RPM needle on your dashboard to the green range when the light turns green. This gives you a boost of the line and also earns you a few easy stars.