This achievement can be earned before you complete the first mission. KUBUS are floating squares that act as the optional collectible in this game, to collect it you simply need to walk over the square. They are scattered around the environment and sometimes require that you expand the level by touching Orbs to build a pathway to them. A video guide detailing the location of each KUBUS (as well as how to complete the game) is below!
First mission starts at 1:30