Hanatsuki's Pieces
Diary
Enter the pink door. Go southeast until you see a red book resting among some trees. Interact with it to receive the Piece.
Wheelbarrow
Enter the pink door. Go southwest until you see a brown tunnel surrounded with sunflowers. Going in it will bring you to a teleport maze. After crossing the bridge, go inside the eastmost tunnel. Then, go inside the tunnel northeast to the one you came out of. Interact with the wheelbarrow to get the Piece.
Raincoat
Enter the indigo door. Find a frog that gives you the Raincoat Piece. (There is more than one.)
Ball
Enter the indigo door. Find a pink cloudy hole that takes you to some pink clouds. Go left and you'll be taken to some pink window doors. Go left again and you'll be in a red window room. Go in the middle of the three doors and interact with the huge bouncing ball to receive the Piece.
Unlocking Tamashitsuki
(The Raincoat piece is needed.)
Follow the Wheelbarrow Piece guide above to get to the Sunflower Tunnel Maze. Take the eastmost tunnel once you cross the bridge. Find an obscure tunnel surrounded by a cluster of sunflowers and a few rocks. Put on the Raincoat Piece and press Shift. All the rocks will come alive and one will move out of the way of the tunnel, allowing you to enter. Interact with the pole to get to the Downtown area. Make your way south through the forest, past the town, and into a forest again. Find a secret path (shown in the picture) and enter it. Move a cone by interacting with it. Avoid the chasers as best as you can through the path (you will have to jump over a hole by interacting with it at one point). Afterwards, make your way up the path and interact with the girl. After the event, go back to sleep and interact with the girl again in the nexus. Interact with the mirror to start playing as Tamashitsuki.