From the main menu head to Championships.
There are multiple event types here but the one I recommend would be the Sprint Series. This consists of 6 5-lap races with no Qualifying sessions.
Make sure to set the AI difficulty to 0 (Very Easy).
your goal now should be to win every race you compete in, and keep an eye on the Championship Standings after each one. If you ever reach a point where you have a 25 point lead over the second-placed driver, you can skip the next race by starting it, pausing the game and selecting "Retire from Session".
Once you've reached the end of the Championship you'll get your Gold Star and your Achievement.