While we strongly discourage using the disposable Baofeng HTs such as the UV-5R radios for repeater use, they are suitable for things where only one radio is needed such as an APRS node.
To help with putting this type of setup together we have put together the following pin diagram. We do our best to keep these drawings accurate, but on occasion we are still human and do make mistakes. If you find such an error, please let us know so we can fix it.
We recommend using this cable which is available through ebay and seller phutping. We like this cable as it has all signals from the HT brought out while many of the cables on the market do not expose all the cables. Additionally by not being coiled, it is far easier to cut to the right length for custom installs. https://www.ebay.com/itm/Audio-breakout-cable-APRS-datamodes-EchoLink-Baofeng-UV-5R-Kenwood-etc-6-core/254179460447
You will notice the Baofeng speaker audio is connected to 2 signals on the Pi-Repeater interface; This is because there is a ~3.3Vdc carrier present when the Baofeng speaker signal is being driven. This dc carrier is absent when the speaker signal is quiet. As the dc carrier signal behaves like an active high squelch signal, it is being exploited to act as the squelch signal going to the pi-repeater controller. This explit allows you to get away from using VOX which is always a good idea.