I also applied another outside coat of epoxy, which made it shinier and smoother. The result is what you see in the photo shown here. This method makes a nearly indestructable adapter, but it may not be worth the amount of effort it takes. You also risk messing up your adapter if you accidentally get epoxy too far down the metal part or in the end. The specs say both the input and output are at line-level.
Most stereos have line-level connections on the back.
Just wire the output of the adapter to an input on the stereo, and an output from the stereo to the input of the adapter. Go into the Prefs on the Newton to set the input and output sources. Don't turn the recording level too high, or you will get distortion. It seems to work well with the record level turned about halfway up. You can't hook a microphone directly to the adapter unless it's a line-level mic with an external power supply.