when tuning a harmonica it is best achieved by a process of small adjustments working slowly and methodically in tiny steps up to the point of perfection and in some cases one step over and one back to truly find the sweet spot although a daunting task tuning is a simple job that when understood will give you a greater satisfaction and understanding of the instrument.
there are three steps to getting a nice even tone from your harp,
1/ AIR FLOW passing through the reed slot and out through your hands
2/ VIBRATION of the reed at a required frequency to achieve a sound unhindered
3/ the condition of the materials used in the instrument to achieve the above
one tool that is a must when tuning is a reference to work to, this can be a harp you know to be in tune
or a chromatic tuner they range in price from £10 to £200