Once you complete all 15 levels of a world you'll unlock a new skin. Stick with World 1 until it's all complete and this shouldn't take long.
Once unlocked return to the World Selection and you'll see this new skin in the World 1 start area, simply roll through it and the trophy is yours.
Just for reference you should get a new skin for completing each world and, apparently, a further skin for each world completed entirely under Voodoo time.