Step 1:
Prefold in half, then fold both edges to the centerline.

Step 6:
Both folding lines are a bit off the centerline.

Step 5:
Repeat step 4 six more times, the mountain-folds are prefolded.

Step 2:
Fold both edges to the centerline.

Step 4:
Make a pleat by folding over these two lines, the mountain-fold is a prefolded line.

Step 3:
Fold to the centerline and unfold the model.

Step 10:
This is how the model looks now.

Step 8:
Glue both edges together into a skirt.

Step 9:
Pull outwards, see pink arrows.

Step 7:
Fold over this line.

Step 12:
Fold this part flat, do the same on the other side.

Step 11:
This is the model from aside, fold flat. Do the same on the other side.

Step 13:
Fold both corners inwards. Make the same folds (valley-folds) on the backside.