You will need:
1. Print out the image or create your own.
2. Hold the paper up to your face, so that the cross is in front of your right eye.
The picture of the 'target' should be to the right of your face
3. Close your left eye. Focussing on the cross, slowly move the paper away from your face.
4. As you move the page away from your face, you notice that at some point you cant see the 'target' within your peripheral vision; you've found where your blind is!