The term "mouth organ" applies to an instrument that is played by blowing air through a collection of tubes or pipes. Usually, when someone says "mouth organ," he is referring to an everyday harmonica. The mouth organ is a simple instrument, but it can be hard to play well. Many people are able to achieve a rudimentary level of playing skill fairly easily, but it takes time and practice to become a skilled player.
Locate the side of your mouth organ with open holes. Blow into the single hole that is furthest to the left. If this is the lowest note, you are holding the harmonica correctly. If this is the highest note, turn the harmonica over, so that the lowest note is on the left side.
Check to see if there are numbers etched above each hole. On most harmonicas, there is one number above each hole, from 1 to 10. If there are numbers on your harmonica, skip to step 4. If there are no numbers, go to step 3.
Stick a thin strip of masking tape lengthwise on the mouth organ, just above the holes. Using your pen, write the numbers 1 through 10 on the masking tape, writing one number above each hole, thereby labelling each hole with a number.
Read your harmonica tab. The tab, which is a simple type of sheet music for mouth organs, should have positive and negative numbers from 1 to 10. For instance, a 6 on your harmonica tab refers to the sixth hole from the left on your mouth organ. A positive number indicates for you to blow into that hole. A negative number indicates a "draw" into that hole. "Drawing" on a mouth organ is when you suck in through a hole, as opposed to blowing through it.
Continue to follow the harmonica tab indications to play the entire song of your choosing.
Part of learning the mouth organ is experimenting. Try blowing or drawing on more than one hole at a time to make chords.
If you share your mouth organ, clean it with soap and water to prevent transfer of germs.