After Zant attacks, you go to the Sacred Grove to seek the power of the Master Sword.
Follow Skull Kid
Look for the stone near the entrance to the Sacred Grove. Howl at the stone to make Skull Kid appear. Skull Kid will summon some puppets, and will make a wall disappear. Kill the puppets and follow Skull Kid. More puppets will keep appearing as you go along, so just kill them as needed. You can listen for Skull Kid's song to find where he went. Attack him each time you find him. If you reach a dead end, backtrack and look for an alternate path. The paths through the forest will change, so backtracking may lead you to different paths that weren't there before.
Eventually you will reach a stone circle, and the Skull Kid will summon more puppets. Kill them. Then you need to wait for Skull Kid to blow his horn before you attack him. Repeat this until Skull Kid opens the wall. Go through to reach a new area.
Stand on the Triforce symbol and howl. The statues will come to life and will move from their original positions. You need to move them back to where they were first standing. They will jump when you jump. So jump to the right, then down, then left, then left again, then up, then right, then up, then up again, then right, then down, then down again, then left, and finally up.
Go through the door that opens, and go straight ahead to find the Master Sword.
After the cutscene, you can warp to Hyrule Castle Town, ask Midna to transform you into a human, and go to Telma's Bar, where you can meet Shad and Ashei, and reunite with Rusl. Look at the map to see where Auru went.
He's at Lake Hylia, so warp there, and climb the stone tower in the southern area of the lake. Say Yes when Auru asks if you plan to go to the desert. Auru will give you Auru's Memo. Go to the tall pink building and set the memo to a button, then press the button to show the memo to Fyer, then get into the cannon.