How to make an IR (infrared) filter.?

That hot mirror in your camera renders infrared photography difficult to impossible. There are services you can send to and get it replaced with one that will work with IR photography. It is better to just buy the filter. They are very dark red, nearly black. They cut down a lot of light, so you will need a tripod.

This site has a lot of good information on IR photography, and offers the conversion service. http://www.lifepixel.com/

The filter size depends on which lens is attached. The front threading of the lens, designated by a O with a diagonal line through it. The other issue is that your camera might not be capable of shooting IR. I know Canon digital cameras have a protective layer on the sensor that blocks IR light, this makes IR photography not possible. I assume Nikon cameras are set up the same way.

IR is filtered using Red.

You do mean to remove IR?

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An IR filter is one of those things that is made to exacting standards of precision. I'd recommend buying one instead of trying to make one.

There's not really anything that will work for you -- floppy disks and film are about it.

If you've ever had film developed and still have the envelope, check and see if you've still got the strips of film. Chances are, they're still in the envelopes, and at the end of one of the strips you should have a couple of blank ones.