What you need
DSLR or point and shoot digital camera.
White typewriting paper
Piece of masking tape
You need to cut the type writing paper like a size of a credit card.
Pop you digital camera's built in flash.
Now you need to attach the piece of paper you cut with masking tape. On portrait orientation, attach the upper edge of the piece of paper on top of the built in flash. Then attach the opposite side under the camera flash.
Now your camera is ready, just set the flash to auto mode or TTL and start shooting!
Here are some test shots. I placed my friend Bob 1 inch away from a white wall to see the difference with and without the improvised diffuser.
Picture on the left hand side is shot with a straight flash and created a harsh shadow behind. The picture on the right is taken with a pop up flash with two layer of paper attached.
Place your subject at least three to six feet away from your background to make your subject's shadow softer.
The pop up diffuser can reduce glare from metallic and shiny surfaces.
You can also use it for macro photography.
And portraits too!
Give it a try, experiment and explore. Try different camera settings with your improvised pop up flash diffuser now.