Step 1:
Tape/glue two black a4-size papers or three square papers together.
Lay it on a piece of tin foil.
Fold the tin foil around your large black paper.

Step 6:
Fold the edge to the centerline.

Step 5:
Prefold in half over this line.

Step 2:
Unfold the tin foil and remove the black paper.

Step 4:
Tape the tin foil to the black paper.

Step 3:
Fold over the prefolded lines.

Step 10:
Repeat step 8 into the other direction, all lines are prefolded.

Step 8:
Valley-fold over these two lines, the corners are pulled upwards when you do this.
Press both parts flat.

Step 9:
Unfold the folds of step 8.

Step 7:
Mountain-fold over the line on the right and valley-fold over the line on the left.

Step 16:
Close the model, lay the upper edge over the edge below, see blue arrow.
Use tape or glue to connect both edges.

Step 15:
Prefold over these four lines.

Step 12:
Valley-fold the edge on the right of the purple line to the left.

Step 11:
Turn the model.

Step 14:
Prefold in half over this line.

Step 13:
Press these two places flat.

Step 17:
Make the same folds (=valley-folds) on the layer below. Use tape or glue to connect both edges.