This achievement will accumulate over time. After every match you play (win or lose) you will receive an item drop for your vehicle. These items range from new vehicles, flags, trails, toppers, etc.
Simply play/participate in 50 matches and this achievement will be yours.
To speed the process of unlocking these items up you can use Exhibition.
Load up an Exhibition game with these settings:
Arena/Map: Any
Team Size: 1v1
Bot Difficulty: No Bots
Mutator Settings: Max Goal = 1
Now you can pick any team you want (it will only be you) and simply make a goal which will end the match immediately. This method is very fast and can unlock an item every minute or two.
This will also unlock these achievements:
Here's a visual reference as to how easy and fast it is to create and finish an Exhibition map for this achievement: