The usual way to make 3D glasses is to tape red and blue cellophane to paper frames. If you don't have cellophane, however, you can easily make the red and blue lenses with a thin sheet of clear plastic--for example, from a three-ring binder insert--coloured with red and blue permanent markers. Replacing the paper frames with plastic sunglasses creates a sturdier, longer-lasting pair of 3D glasses.
Pop the lenses out of the sunglasses.
Trace the lenses on the clear plastic.
Colour the right lens blue and the left lens red using permanent markers. Make sure you fill in the entire outline and don't leave any clear spots.
Cut out the lenses and place them in the frames. Use tape to keep them attached, if necessary.
If you don't have sunglasses, print out a pattern for 3D glasses frames. (See the U.S. Geological Survey website below for a pattern. This is a good size for children; for adult-sized glasses, enlarge the picture slightly before printing it out.)
Glue the pattern to heavy paper, such as a paper grocery bag, or cardboard, such as a cereal box.
Cut out the frame and earpieces and tape or glue them together.
Colour rectangles of clear plastic with blue and red permanent marker and tape them into the frames, as above.
If you wear glasses already, just place the coloured red and blue pieces of plastic over your lenses and attach them to the frames with a small piece of tape. You can also create simple 3D "glasses" by colouring the right half of a clear CD case blue and the left half red.
Make sure the coloured lenses are on the correct sides; otherwise the glasses won't work.