As you play the game and go to one of the many regions in the world map, you will find these skull icons on your radar that indicate there is a ocularum there. Go up to it and press A to start a search for the shards in the area. Its a minigame where you move the right stick to locate the number of shards it tells you to find in that area. Once one is identified, head to that area and you will see an icon for the shard show up on your minimap. Once you pick it up, the achievement will unlock.