At first sight this Achievement seems Daunting but actually its quite simply when you know how.
The Best and Easiest way I have found to do this is to Max out your Cleaning Skill and Replay the "Cleaning the garage" Job.
To replay any Job go to the "MAIL" tab on your laptop and at the top you should see an "Archive" tab. Click on the tab and you should see all your previous Jobs that you have completed
NOTE: You DO NOT need to 100% the mission for this achievement to unlock!