Step 1:
Make a valley-fold over this line.

Step 6:
You've made all prefolded lines now, but many of them are in the wrong direction. Start making a valley-fold on the left. Make a mountain-fold over the line next to this first line. Continue making valley- and mountain-folds until the folding line completely on the right in this picture.

Step 5:
Unfold the model.

Step 2:
Make a valley-fold over this line. Fold over both layers of the model.

Step 4:
And fold the model one more time in half over this line.

Step 3:
Make a valley-fold over this line. Make this fold over all layers of the model.

Step 10:
Fold the piece above the purple line downwards by making a valley-fold over this purple line.

Step 8:
Unfold the model again. Then, turn the paper, with the green side above.

Step 9:
Fold the piece below the purple line upwards by making a valley-fold over this purple line.

Step 7:
Here a part of the folds from step 6 is already done.

Step 16:
Make the folding lines of the green part sharper. These lines are already prefolded in step 1 to 8.

Step 15:
Press the end indicated by the arrow a little further in the opening of the other end (In the direction of this arrow).

Step 12:
Fold the piece below the purple line upwards by making a valley-fold over this purple line.

Step 11:
Fold the piece below the purple line upwards by making a valley-fold over this purple line. Make this fold over the upper layer of paper only.

Step 14:
Hold the model the same as I do in this picture. Insert the corner indicated by the arrow below in the opening at the upper arrow. Make sure that the folding lines of both ends fit together well.

Step 13:
Roll up the model.

Step 17:
Press the 2 layers of paper at the large arrow against each other and lock them together by making a mountain-fold over the short white line. Make this fold over both layers of paper. Make the same fold on the corners indicated by the small arrows.