Since Monaco is obviously the best circuit to trigger the Safety Car, set up a Grand Prix there and make sure you have selected 20% or more in race distance, since the Safety Car can only be triggered in 20%+ races.
Set Safety Car to ON and start the race.
I found that triggering the Safety Car works best at the hairpin (turn 6). Simply stay back a little and when the time is right, accelerate through the middle of the pack and put your car into the hairpin crossways - since no car will be able to pass you a massive line will form, therefore triggering the Safety Car.
When you trigger it a yellow box saying "Safety Car deployed" will show up in the top middle of your screen. As soon as this happens all you have to do is perform a pit-stop and the achievement will pop.