Step 1:
The size of this paper is 20x60 centimeters. It's made of 2 standard printer-/copy papers glued together with aluminium foil folded around it. You can also glue three squares together.
Prefold in half

Step 6:
Fold in three parts of the same size.

Step 5:
Fold the edges to the centerline one more time.
Then unfold the model.

Step 2:
Fold the edges to the centerline.

Step 4:
Fold the edges to the centerline.

Step 3:
Fold the edges to the centerline.

Step 10:
Repeat step 9 over the whole length, the mountain-folds lay 2 squares apart.

Step 8:
Fold in half and unfold the model.

Step 9:
Now the paper has a prefolded pattern of 32 by 12 squares which is used as guidance for the next steps.
Fold over these two lines, the mountain-fold on the right is already prefolded.

Step 7:
Fold in half.

Step 16:
Now the model has a pattern of 16 by 5 small mirrors.
Fold the spots indicated by the arrows against each other. These spots are in the middle of a mirror edge.

Step 15:
Repeat step 13 and 14 in the other direction.

Step 12:
The mountain-folds are already prefolded.

Step 11:
Repeat step 9 and 10 in the other direction

Step 14:
Repeat step 13 four more times.

Step 13:
Mountain-fold over the prefolded line below and valley-fold over the upper line

Step 21:
Glue / tape the edges together

Step 19:
Fold both corners backwards, do the same on the other end, see large arrow.

Step 20:
Fold both corners backwards, do the same on the other end.

Step 18:
Repeat step 16 and 17 two more times.

Step 17:
Fold these spots against each other, they are on a corner of a mirror.

Glue / tape the edges together

Step 22:
Fold the upper and lower edge downwards.