Summarize:

One type of 3D glasses commonly used in theaters uses polarized filters for lenses, and special projectors that polarize light. Think of the polarizing filter as a barred window: light oriented (polarized) vertically passes between the bars and reaches your eye, while light oriented horizontally can't get through the bars and gets reflected away. With the "bars" over each eye pointing in different directions, each eye picks up a different image, and your brain interprets the two images as a single 3D image. Unlike the red-blue glasses, this image can contain any number of colors. Making these glasses at home is probably more expensive than buying a pair, especially since any theater experience or TV that relies this technology probably comes with a pair of glasses. But if you're interested in the project, purchase a sheet of "linearly polarized" or "plane polarized" plastic film. Rotate the film 45º from the vertical, then cut out a lens. Rotate the film another 90º in either direction, and cut out the second lens. This is the most common design, but you may need to rotate the lens while looking at the 3D image to see what works. Just make sure to rotate both lenses at the same time, since they should always be made from film oriented exactly 90º apart. The actual explanation of polarized light is more technical than described above. Modern 3D glasses usually use circular polarized light, which doesn't require the viewer to keep his head still while watching. To make these lenses at home, you would need one sheet of counterclockwise circular polarized plastic, and one sheet of clockwise polarized plastic (also called left- and right-handed). This is more expensive than linear filters. Sometimes called "Active 3D," this technology requires advanced design that can't be copied at home. In order to send a different image to each eye (which is the cornerstone of all 3D technology), the television monitor switches rapidly between two different images, many times per second. The special glasses you wear while watching are synchronized to the television, and each lens alternates between dark and clear at the exact same time, using tiny liquid crystal cells and an electrical signal. This is considered one of the most effective 3D glasses for comfortable, long-term use, but it's just not feasible to make these in your basement, let alone the television set programmed to synch with them.
Learn about polarized glasses. Make your own polarized glasses. Understand synchronized glasses.