This is unlocked by equipping any character clothing and entering a world. Clothing is earned as free gifts by simply playing the game as well as gifts for returning to the game. I'm not sure what the frequency is but I seem to earn them about every 3 days real time and about every hour of in game play time.
The easiest way to do this is after unlocking some clothing start a new world, during character selection you can customize your character, equip the clothing then start the world and the achievement will pop.