Step 1:
Tape / glue three squares together first.
Fold in half.

Step 6:
Fold the edges to the centerline.
Then unfold the folds of step 5 and 6

Step 5:
Fold the edges to the centerline.

Step 2:
Fold over these two lines.

Step 4:
Prefold in half

Step 3:
Make the same fold on the backside.

Step 10:
Repeat step 8 and 9 with this part

Step 8:
Pull the left part upwards.

Step 9:
Press inwards from aside.

Step 7:
Make zig zag folds over the prefolded lines.

Step 16:
Fold the 12 upper edges downwards, see yellow lines as example

Step 15:
Glue these two edges to the edges on the backside

Step 12:
And repeat step 8 and 9 here

Step 11:
Repeat step 8 and 9.

Step 14:
Pull / fold these two edges backwards

Step 13:
Glue this edge to the edge on the backside