Step 1:
Fold over the purple line, it's about 4 centimeters away from the edge.

Step 6:
Fold over this purple line

Step 5:
Fold the four prefolded mountain-fold lines 2 centimeters to the right, use tape or glue.

Step 2:
Prefold in half, then prefold both edges to this prefolded centerline.
Unfold the fold of step 1 afterwards.

Step 4:
Prefold in half, then prefold over the lower purple line. The distance to the edge of this lower line depends on the size of your lamp holder.

Step 3:
Prefold each part in half.

Step 10:
Insert a lamp holder and screw it to the lampshade

Step 8:
Fold this corner downwards.

Step 9:
Fold the other 7 corners downwards.

Step 7:
Make sharp folding lines on the eight edges and glue/tape the right end to the left flap, see purple arrow.

Step 12:
And screw a real lamp in the holder

Step 11:
The video shows how I made the small cover for the lamp holder.

Step 13:
This is how the inside of the lamp looks.