Step 1:
This a4-size paper is yellow on one side and white on the other side. Other color combinations are fine too.
Mountain-fold over this line, it's just a little bit below the centerline.
Then turn the paper, see next picture.

Step 8:
Valley-fold over the short blue line.

Step 6:
Press the edges indicated by the two arrows against each other.

Do the same on the other two same places.

DONE!

Step 12:
Turn the model.

Step 11:
Push/press these spots upwards too

Step 10:
Push/press these edges upwards with your finger.

Step 9:
Repeat step 7 and 8 on the other two corners.

Step 5:
Fold these two prefolded lines into the right direction

Step 7:
Hold the corner indicated by the arrow between your fingers, see next picture.

Step 3:
Valley-fold over these two lines.

Step 2:
Prefold in half over this line.
Then turn the paper again.

Step 4:
Valley-fold over these two lines, then unfold the folds of step 3 and 4

Step 13:
Slide the end on the right into the opening on the left. Use tape or glue to hold both parts together (if necessary).