If you pull out the SA-21 from your scanner, you'll see a female electric contact for about 20 pins. You need to short two of them (don't remember which ones but you should be able to find it with google). If you know how to use a soldering pen, you can do it on the circuit board, otherwise use a hair-thin wire and a piece of scotch tape to keep the wire in place. I used a single copper wire from a multicore speaker cable or something similar, it doesn't matter as long as it is very thin. Then just insert the film holder into the scanner again and open the little hatch on the back to allow the film to run through. Power up the scanner and run your scanning software as usual. If you're using the Nikon scanning software, it will automatically recognize your modified SA-21 as a SA-30 and allow you to select up to 40 frames to scan. (hint: preview scanning a whole roll takes a lot of time and RAM!)