1. Take a new file of 400 pixels,400 pixels, of resolution 72 dpi in the RGB mode.
2. Fill the new document with color black.
3. Now take the polygonal lasso tool and create a diamond shape which I am showing in the image below
4. You will need to fill it white then deselect.
5. Go to Blending Options apply these settings to give it a real look.
6. Your final image will be as attractive as mine as showing below.
7. I have put it down it on the wallpaper.