I know this is not something new- Just want to share my experience making one Fader ND filter using 2 CPL filters.
DIY faders are done by reversing the side of one CPL filter and attached it to a normal CPL filter. However, as most CPL filters are quite thick, the resultant "product" will have double the thickness- not suitable for wide angle lenses, even the kit lenses starting from 28mm (35mm eq). I personally have made one with this method.
To reduce the thickness, I made use of a modified "UV filter" to replace the CPL filter nearer to the lens:
Step 1: Buy the filters
I bought the CPL filters and a UV filter (72mm) from eBay. They are very cheap (made in China) but the quality is quite decent:
Total cost = $50.17 (with free shipping, received within 3 days), much cheaper than the China made 72mm Fader ND filters.
Step 2: Disassemble the UV filter
This is the most difficult part- I do not have the lens spanner so I have to carefully turn the holder ring anti-clockwise with a small screwdriver. If you are not careful, you might scratch the surface of the filters... OR even worst, hurt your fingers. I was quite lucky . Better to get a lens spanner.
We do not need the glass from the UV filter so just keep it aside safely in case you decide to reverse the whole process.