You are required to complete a race in the 1998 Porsche 911 GT1 Strassenversion on Maple Valley.
You are not required to win the race, just complete it.
To start off head to Single player > Freeplay.
Hit RB to go over to setup.
Select Car: (If you do not own the vehicle simply go to the rent option)
1998 Porsche 911 GT1 Strassenversion
Race Setup:-
Environment: Maple Valley
Track: Short Circuit (or full if you would prefer)
Laps: 1
Max Drivitars: 23 (default)
Scenario: Day or Rain**
Car Division: Any
**Set the scenario to rain and you will also unlock this achievement as soon as the race starts (if you haven't already unlocked it)
Complete the race and the achievement is yours.