Above is a picture of the cable that I built. It's only about 25cm long, enough to get the RS-232 female socket to the outside of the Apple ][+. From there, a normal RS-232 cable connects to the PC.
I've used ribbon cable to join the two connectors, as it keeps it tidy and each wire is colour coded. Note that one resistor connects pins 4 and 8 on the 16-pin header, and the second resistor is in series with the diode. Note the orientation of the diode: the `bar' on the diode is towards the second resistor. Three wires connect to the diode, pin 8 and pin 15.
On the RS-232 socket, the three wires connect to pins 2, 3 and 7. I've used
the remnants of the resistor legs to join pins 4 & 5, and 6, 8 & 20