●部分を平たい面にする(A)

Make the ● part flat (A)-1

Open the folds marked with ● in the left image, flatten the folds, and create a flat surface like the one in the right image.
An example of the specific steps is shown in the following image.

1 Stand the front right piece of paper up in the center.
2 Hold the two folded edges of the paper with your fingers, overlapping each other, at the ▲ part on each side, and slowly pull the paper left and right to open it.
3
4 Once the paper is fully opened, align the left corner B with A and fold the left CD.

5 Make a crease in the CD and return it to its original position.
6 Fold the right side of the CD, aligning the right corner B with A.
7 Make a crease in the CD and return it to its original position.
8 Make a mountain fold crease at DD.
9 Make mountain fold creases at AD on both the left and right sides.
10 Pinch the ▲ parts with your fingers on both sides and move them towards the center, and the paper will fold naturally.
11
12
13 Make the ● part flat (A) is complete.

● Flatten the area (A)-2

In (A)-1, we were able to create one mountain with a flat surface. Next, we flatten all the other mountains as well to create the shape shown in the right diagram.

1. Fold the front left piece of paper to the right.
2. Fold the top diagonal edge of the front left piece of paper to meet the center crease.
3. Fold along the crease made in step 2.
4. Unfold.
5. Stand the front left piece of paper upright in the center.
6. Using your left and right fingers, hold the two folds at the ▲ sections of the left and right pieces of paper, overlapping them, and slowly pull them to the left and right to open the paper.

7 With the paper not fully open, make valley folds on the left and right CDs.
8 Make mountain folds on DD.
9 Make mountain folds on the left and right ADs.
10 Pinch the ▲ parts on the left and right and bring them towards the center, and the paper will fold naturally.
11
{Flip}
12 Fold the mountain on this side in the same way, making all the mountain sides flat.
13 Finished.