To create a custom tuning, go to the Settings page and tap on the New Tuning button. This will clear out any tuning data from a previous tuning. It will also automatically select the note C1 for you and select the Measure Inharmonicity button on the main tuning screen. When you hear VoiceOver announce the Measure Inharmonicity button, double tap the screen to activate that button and begin a measurement and immediately play C1.
After you play the note and the measurement period is complete, which happens in under six seconds, a result screen will be shown that displays details of every partial detected, especially the offsets for each partial. It is not necessary for you to review that data. It is presented only in case you want it. The software automatically uses those offsets to calculate the inharmonicity constant for that string. That inharmonicity constant is spoken immediately by VoiceOver when the results screen is shown. If that number sounds reasonable to you, just activate the Save button, or use the two-fingered Zee-gesture, which will save that reading and return you to the main tuning screen, where you can measure another note.
After every successful measurement, the selected note will be advanced to the next C note automatically, and the Measure Inharmonicity button will be selected. Because of these operations, you can quickly step through the measurement of C1, C2, C3, C4, and C5. You can manually select other notes too and measure them if you wish, but these five C notes are usually sufficient for Montal Tuner to calculate a good custom tuning, and measuring additional notes will make no difference.
After every inharmonicity reading, the software recalculates the stretch. When you are done making a custom tuning, you can either proceed to use it immediately to tune the piano, or you can save the tuning on your device so you can reload that same tuning the next time you tune this or a similar piano.