The global championship consists of 6x5 races, or 6x7 races, depending on your choice. If you just want the achievement, just do the 6x5.
The 6 stands for the different countries where you race.
The 5 stands for 4 rally races and 1 crossover.
The 7 stands for 5 rally races and 2 crossovers.
Choose a class like classic cars , fwd, rwd or 4wd, and choose the best in the class. Good for this is either the Lancia S4 or Colin Mcrae R4.
Of course, you're able to change difficulty, so just choose rookie if you want a simpler achievement.