Basically when you go to Kuchihagahara you are looking for a small green-ish coloured bag on the ground. It appears in several area, I'd suggest start the search in the north end of the river, go down to the path to Amana and if you still can't find it follow the east side path up towards Minori Castle. It's always been near a wall for me.
When you find the item you have to get back, if you head directly to Takatane a crow will appear and steal the item if it makes contact with you. To avoid it you I generally need to go in a circle around it.
Alternatively you can just go to Amana or Minori and quick travel back to Takatane and not even worry about the crow.