Hold the phone near the guitar. Pluck the lit string and let it ring. Turn the peg the way the app says until the line turns green, then hold it there for a moment. Resonant locks the string in and lights the next one. Six strings later you are done.
Move the phone closer to the guitar, pluck one string at a time, and let it ring rather than muting it. Fans, air conditioning, and loud rooms make a string harder to hear, especially the two thinnest ones.
Tap the string you want on the guitar drawing and Resonant switches to it.
Tap the chip at the top left for guitar tunings (Drop D, half step down, DADGAD, open tunings and more), bass, and ukulele.
The watch app tunes on its own, with no phone needed. It asks for microphone permission the first time you open it.