Grab yourself a friend (or in my case a second account playing on a separate laptop) and go to 'Multiplayer' and 'Create a server'.
Make sure to go to the option and do the following;
- Set a server name
- Set a password
- Event type to Banger Race
- Change the max players to 2
- Number of laps to 1
In the select a track, choose a short looped track. I personally chose "Mudford Motorpark" oval.
When you're both in the game and cars selected, start the match, the player going for the win has to complete the one lap whilst the other player can simply "retire" from the menu, or can too complete the lap (if you're soloing it's quicker to retire).
When you've completed, return the lobby and restart the race, simply repeat this quick win another 19 times and the achievement is yours. It should take about 15 minutes of boosting to get the 20.
NOTE: The race must be played with another "Real" account, bots do not count towards the win progression.