Warning, this alternative appears to be more reliable than mine, please visit here and ignore what's written below: https://github.com/johnboiles/BaofengUV5R-TRRS

[caption id="attachment_536" align="alignnone" width="1024"]aprs cable This cable follows the below diagram precisely; note the color-coded heat shrink, and note that the TRRS plug's internal wiring is such that the Tip is wired to the furthest-left solder point, followed by R1 and R2, and finally the Sleeve solder point is actually also the strain relief and threaded housing. So you've gotta think a bit backwards (and/or use an ohmmeter to check your wiring.)[/caption]

Sources:

Warning, this alternative *may* be more reliable than mine: https://github.com/johnboiles/BaofengUV5R-TRRS

And this article has a lot of helpful hints: http://www.wcares.org/?page_id=2677

And of course, using a Bluetooth TNC, etc, is going to be much more reliable than this hack. I don't use this on a regular basis.

Finally, this cable seems to work well on my Samsung Galaxy Nexus, but is not detected as an output by my Nexus 5x when plugged into the radio so it might need different resistor values or connections depending on your devices.

I harvested a cheap Baofeng handheld speaker/microphone for its cable (although you can buy them here -- the Kenwood type) and bought a 3.5mm TRRS plug from Digi-key. It's a bit tight to solder the resistors inside the plug housing, but it is possible (just remember to put the plug housing on the cable BEFORE soldering, otherwise you'll be unable to put it on later... a mistake I made about three times.)

Here are the diagrams: 

[caption id="attachment_535" align="alignright" width="288"]aprs cable Your wire colors may be different; I made this diagram match the wire colors from my cable so I wouldn't get things confused.[/caption]

I'm sure there is some science behind the correct resistor ohm values (and as in the Github link, an additional capacitor might help; I'm not sure, this is just a hack.) I used 480ohm resistors. The idea is to mute the audio slightly because amplified earphone output being wired straight into amplified microphone input is a surefire way to transmit nothing but distortion.

In my experience on a Galaxy Nexus running APRSdroid with this cable into a Pofung UV-82, the best reception is with the radio volume at barely audible (1/3rd of a rotation) and with the phone volume at 1/4. Further experimentation would yield better data, but keeping an eye on received packets and repeats of your transmissions can help you even if you don't have a separate radio to listen on. You can also use a recording app on your phone to see what the phone is hearing.

Helpful settings on radio: VOX at ~2, Squelch at ~1. Ensure that both channels are set to the APRS channel before enabling VOX or connecting cable, and connect the cable while radio is off, to avoid unwanted transmission. Consider listening/testing your transmissions/receptions on a separate radio and looking at the radio's receive/transmit lights to ensure that the phone is being polite and not transmitting over other stations, and is actually audible.