For this, you need to gold medal (1st place) all Amateur Cup races in career mode.
Most of the races are decently short and on the "easier" version of some of the courses, so this achievo shouldn't be to bad.
You will need to be at least a level 35 in order to complete this achievement.This means you will have to race most of the other cups before you can complete this achievement...which means your basically back-tracking to go back and complete the amateur cup. The second to last race of this cup, you will need a v12 car. I got one from doing 1 of the endurance races, or you can try to get one cheap at the auction house.