Installing
Take the build for your system from the downloads section. On Linux the AppImage needs no installation: mark it executable and run it. The deb and rpm packages register the app with your desktop in the usual way. Windows has an installer.
There is no macOS build yet. The Mac port is written, but building and notarising it needs Mac hardware.
Permissions
SmeggTuner needs access to an audio input device. On Linux that is usually nothing more than being in the right group; on Windows and macOS the system asks the first time you press Start.
Choosing a microphone
Almost any microphone will do, and this is the part people overspend on. You are measuring frequency, not recording an album, and a cheap dynamic microphone measures frequency exactly as accurately as an expensive condenser. Sensitivity, noise and tone do not enter into it. A plain dynamic mic plugged straight into the sound card is the sensible default.
Placement matters more than the microphone
Put the microphone on the same side of the accordion as the reeds you are measuring: keyboard side for the treble, bass side for the bass. If two reeds sound together and the microphone favours one of them, the quieter one is measured worse, and that is a real loss of accuracy.
Get it under half a metre from the instrument. Closer means the reed is louder relative to everything else in the room, and that is the whole game.
Turn off the microphone's own processing
This is the one that quietly ruins measurements. Many microphones and sound cards ship drivers with echo cancellation, noise suppression, stereo enhancement, directional processing or their own hum filter. All of them deform the signal, and a deformed signal cannot be measured. Turn them off in the operating system.
Boost, gain, volume and balance are fine. They change the level, not the shape.
Levels
SmeggTuner does not ride the gain for you. It watches the level and tells you when it is wrong: Too loud means the signal is clipping and the reading will suffer, Too quiet means the reed is not clearly above the room. Set the level in your system mixer until neither shows, and leave it.
Hum
If your reading wanders by a hertz or so with nothing playing, that is mains hum from the power supply, picked up by the microphone. It lands near 49, 58 and 62 Hz, so it can even be detected as a note. Turn on the 50 Hz filter (Europe) or 60 Hz filter (North America) in Settings and it goes away.
A first reading
Press Start and keep the instrument quiet for about five seconds. The tuner profiles the room's background noise, and anything playing during that window gets learned as background and then ignored. If you played through it by accident, stop, wait, and start again.
Then play one note and hold it with steady bellows pressure. The reeds need about three seconds. The note strip shows what the app thinks you played, the spectrum shows the tone it locked on to, and a value box appears per reed. When the reading is stable it locks, and the tuner stops jumping to other notes for as long as that note sounds.
Play at the pressure the instrument is actually played at. Pressure moves pitch: lean on the bellows and the reed goes flat.
Where to go next
The tuning guide walks a whole instrument end to end. If you have never tuned a musette before, read the vocabulary on the overview page first: the words carry a lot of weight here, and the rest of the docs assume them.