Smoked Glass, Gray Theater Gel, Tinted Plastic... You need some sort of grayish substance to do the actual neutral density bit. That smoky gray tinted glass could be used, as long as you have the proper supplies to cut it to shape. Gray gel used for theater lighting could also be used, if perhaps a bit harder to get your hands on depending on your available resources. If you can find good quality transparent gray plastic, this might also be an easy way to make one. Whatever you use, make sure that there is enough of it clear of scratches to do the job.
As you can imagine, the darker the glass you choose, the less light that will be let through the lens, so test the glass before purchase to make sure it's as dark or light as you want it to be.
Also, make sure that the material is gray—hence neutral density filter. Any glass that is slightly colored will, well, slightly color your image. This may in itself be a cool effect, but it's not what we're covering in this DIY article.
Alternatively, if you have an old film camera handy, you can always expose film to varying degrees and use that for a filter medium.
Cutting Utensil. Depending on the material you chose for the filter itself, you'll need a different cutting utensil. If you're using glass, then a glass cutter is really your only solution to get a clean cut. Otherwise, if you're using plastic or theater gel, a box cutter will probably do the trick.
Sandpaper. Chances are, your edges won't be too clean, and they'll need to fit your filter mount exactly.
Lens cloth. You'll need something to clean off the future filter during the construction process to keep it clean.
Glue. You'll need something strong that won't puff up too much when it dries, as Gorilla Glue will. Hot glue if you're using plastic materials will do especially well.