After completing the mission "Number 1" for Brucie, you unlock the ability to compete in street races. Brucie will call you on your phone soon after the mission is over, and explain that any time you want to race, give him a call to set one up. Win 20 races to unlock the achievement.
Now, initially you only have the option to do 9 races, as is the total required for 100%. You need to be standing on the 1st island and call Brucie, he will then set you up for one of the three possible races on the 1st island. Once all three are completed, he will tell you that there are no more. You then need to travel to Algonquin, call him, win those three races, then go to Alderney, call him, and win those three races.
After that, he will send you a text message telling you that you won all the races, and can call him back whenever to race any of them again. At this point, you have completed your quota for 100% completion, but still need 11 more for the achievement. At this point, they work like every other friend activity. Sometimes he won't answer, sometimes he won't know of any, and sometimes he'll set you up with a race.
**Can be done after completing story mode**