Although "Demon" is Level 7, we are going here before "Lizard" due to its difficulty.  Demon is one of the easiest labyrinths in the first quest.

The journey to Demon, though, is a bit different than the last two.  First, get at least 100 rupies, food for a Hungry Goryia, and make sure you got medicine and bombs.

Now, go to the farthest west fountain in the overworld and blow your whistle.  When that happens, the fountain will dry up and the passage will open.