The Goddess Cubes and Chests are new items to the Zelda series introduced in Skyward Sword. When Link activates a cube with a Skyward Strike, the corresponding chest in the sky will activate and light up, allowing Link to open it.
Tips: build the top of the chest first, then add the bottom piece by piece. The finished chest should just sit on the four legs, or you can glue them down. Remember, you can refer to the .pdo file for help.