This is a very simple achievement, and only requires either
A) you rock at decal designs
b) you have a friend who owns the game.
Head over to the paint shop on the festival ring, and enter the vinyl section.
Make any vinyl, using various layers and then save it using a keyword. If your boosting, just use your Gamertag as the keyword.
after you save it, you will get the option to upload to the storefront. Accept the offer and upload your creation.
Make sure you set the vinyl to SELL not SHARE. Set the price to 1k.
Then just message your friend telling him to purchase it. They can find it using your keyword to search.
Once that's done, hit start and go to the message centre. You'll have a message informing you of your new credits. Download these credits from the message, and the achievement will pop.