For the ZS6 around 75 ohms serial is ideal.
For the ZS5 you would want a voltage divider though, 16+1.5 (Ear Buddy) is what I use.
And if those diagrams are confusing to you, the construction is dead simple... resistors in series connected input-16-1.5-ground, pull your output signal line from inbetween the 16 and the 1.5, ground just goes to ground. Just make sure to try to cut everything to exactly the same size, those lead lengths matter.
Oh, and connect everything directly to the input jack if you can, don't use extra wire between the input jack and the resistors, it will help give you better channel matching. You can add more wire on the output signal leads if you need space/length.