So you will need a piece of paper, a cotton ball, some kind of cheap oil, and a place for it to dry.
They will put the oil on the paper using the cotton ball.
They will need to cover the whole paper.
You need to put this somewhere where it will dry, then you will do the same thing to the other side of the paper.
As you can see, you can now see through this paper (the one on the left) with the oil on it. Light will now come through it, unlike the one on the right!