Update: The April game update makes all of the initial 200 cars that launched with Forza 5 available as rental cars so it's no longer necessary to use the "rental glitch" if you don't own the car that is required for this achievement.
Race #1
Go to Free Play
Car: 2013 McLaren P1 (R Class)
Track: Silverstone - Grand Prix Circuit
Rounds: 1
Drivatar difficulty easy.
If you don't own the 2013 McLaren P1 you can rent it under "Rent Car". Make sure to use the regular edition not the limited edition version.
Race #2
Go to Free Play
Car: 2010 Aston Martin One-77 (A Class)
Track: Silverstone - Grand Prix Circuit
Rounds: 1
Drivatar difficulty easy.
If you don't own the car select "Rent Car" and choose any car (the Aston Martin isn't available there). Then select "Buy Car" and choose the Aston Martin. Before selecting a design press "B" until you're back at the main menu of Free Play. The Aston Martin will be your car now.
Start the race and get the gold medal to unlock the achievement.