This should be the 4th achievement you get.
After coming back from getting the Power Slash technique, you'll run through the River of the Heavens, go through the glowing doorway, and enter back in front of the Sakuya tree in Kamiki Village. Cut the peach to start the Kamiki Village on its way to recovery.
Go through Kamiki Village and you'll see the peach icon on a cliff behind a couple of the buildings. Go across the small bridge across the water, and then enter the house on the left that looks slightly like it has been made out of a wooden boat.
Break the large pot inside, and fall to the basement, and examine the statue on the pedestal. Go back outside. Follow the directional arrow until you get to a large wooden platform.
Make sure you save at the mirror BEFORE continuing on, just in case your achievement glitches. You'll have to reload your save file and do it again if this occurs.
Issun will mention how you need a Sun, and how a sun god could get rid of the darkness.
Luckily, Okami Amaterasu is the Goddess of the Sun! Draw a circle!
We will now meet Village Elder Mr. Orange.
You'll have to battle three green imps.
When you've defeated them, and started moving, this achievement will unlock.