All you need is:
-Oil (baby oil, vegetable oil, olive oil...) We used vegetable oil.
-Construction paper cut in shapes. I noticed that the larger shapes looked better when we were done.
To make this, pour a small amount of oil in your bowl. Dip your cotton swab in the oil, and then draw on the construction paper with it. After it dries, you punch a hole in it and use string to hang it in a window. My 3 year old loved making this because he could do everything by himself and he likes how they look in his bedroom window.