Step 1:
I cut wrapping paper into long paper strips, other papers are fine too.
Fold over the purple line.

Step 6:
Tape/glue the edges together.
Then repeat step 5 and 6 over the whole length of the strip.

Step 5:
The mountain-fold is prefolded and the valley-fold is 1/3 of the distance between the prefolded lines.

Step 3:
Prefold over the whole length of the paper strip.

Step 2:
Use an object of 8 to 15 centimeters to place the folds, I used a stack of sticky notes.

Step 4:
This is the prefolded paper strip.

Tape or glue a few of these parts together.... Done!