There are 2 ways of unlocking this achievement.
To get to this race, simply Select Xbox Live and then Either choose Private Match or Matchmaking.
1. You can use Matchmaking and hope to find a lobby and just complete an Online race and this will pop after the race once u click
2. Easier way, you can select Private Match and invite a friend to boost races and just select which track u want and set it to 1 Lap. If you have 2 xboxs and gameshare with yourself you can also just do it that way as well (your dummy account doesn't have to finish, once u finish the race a 30 second timer will count down) if u boost with a friend it should be alittle faster since you'll probably be finishing around the same time.. Achievement will unlock after you select
to Proceed after the race is finished.
You do not have to win the races so if boosting, you just have to run 20 Races to unlock this achievement and the following: