You will need: I simple printer paper.
or any other sized paper in a rectangle shape.
Fold your paper in half.
Sharp the edge with your nail.
Open the paper.
Now you have a very visible line sharing the paper in 2 long rectangles.
Step 1: Just begin with a simple sheet
Make the fold as showed below.
Very simple, don't need so many explanations.
Do it for both top half sides.