Getting a steady reading
Pluck or play one clean note at a time and read the needle once the attack settles - the first fraction of a second of a plucked string runs sharp before the pitch stabilizes. Mute neighboring strings, close the distance to your microphone, and kill background music. Very low notes (bass E and below) need a moment longer to register; if a low string reads an octave off, trust the note name and the needle rather than the octave number.
Prefer tuning by ear, or teaching someone to? The reference-tone tuners play each string's exact pitch for beat-matching: guitar, bass, ukulele, violin, and the full instrument list - and the tune-by-ear guide teaches the beat-listening skill itself.