For this achievement, you need race cleanly without penalties in order to obtain this rating.
The first thing you need to do is race your 5 placement races which will determine your starting position. The cleaner you race the higher the rank.
Once you have done this and say for example you have a rating of 'B' you will be given a percentage which will increase and decrease after each race depending on your performance. Your target for this is to get this percentage to 100%, after hitting 100% you need to complete 2 more clean races consecutively in order to receive the 'A' rating. Once you have this start up another race and it should pop before/after Qualifying.
This achievement is hard as you can be crashed into and still be penalised so I would advise if you are just going for this achievement stay at the back of the pack and be careful.
Penalties, such as a drive through for a false start will decrease your score significantly so try not to get any of these.
This one could be quite difficult so my advice is to take it slow as the longer you go without any crashes/penalties your rating will increase.