Step 1:
I glued four standard size papers together into one large folding paper.
Prefold in half over the four purple lines.

Step 6:
Fold over the purple line

Step 5:
Prefold over these two lines

Step 2:
Fold over the two lines at the bottom right, then over the two diagonal lines. And then over the two lines at the top left.

Step 4:
Squash-fold.

Step 3:
Fold one flap straight upwards.

Step 10:
Fold the edge in half over the purple line.

Step 8:
Fold the little triangle upwards.
Repeat steps 4 to 8 on the other three flaps.

Step 9:
Pull open

Step 7:
Press the edges downwards.

Step 16:
Glue these two parts together, see arrow.

Step 15:
Pull / curve both flaps downwards, see arrows as example.

Step 12:
Bookfold to the left over this line, do the same on the backside.

Step 11:
Close the model again.
Repeat steps 9 to 11 on the other three same places of the model, see small arrows.

Step 14:
Press inwards from aside.

Step 13:
Fold the two front edges over the two edges on the back, use glue if necessary.

Step 17:
Fold over the two (very) short purple lines and bend / curve the bottom of the banana.