"Hi, my name is Marcus Milius. I play harmonica. I got a degree at the University of Southern California, a Bachelor's of Music, in Jazz Studies - Chromatic Harmonica. I play and teach harmonica here in New York City, and I'm happy to show you what I do.
Playing a 12 bar blues on a harmonica can be as simple as just playing along with accompaniment that is doing a 12 bar blues so as long as you in the same key and you're playing in the correct position, blues can still be played in 2nd position which means you'll be playing in the key of G on a C harmonica, and that's also called a cross-harp. As long as you're playing in a cross-harp or second position with the accompaniment of a 12 bar blues, you'll be playing a 12 bar blues. The form is dictated by the accompaniment and your participation could simply be improvisation or on top of that, it would sound something like this.
So there are 8 bar blues, 16 bar blues, and even three-two-bar form. And so what you want to do is learn to play the roots for each chord just like you would with as a bass player. The root is the lowest note of each chord and so for instance in a 12 bar blues in the key of G, the G chord, also known as the 1 chord, because there are 7 notes in a key typically and 7 chords. The 1 chord in the key of G is a G chord so you would play a G for the 1 chord and that is two draw. And then you go up to the 4 chord which is one blow, the 5 chord and one draw. And the 12 bar blues is 12 bars or measures of 4 beats each that repeats over and over again. Let us divide this into three lines. The first line being 4 bars of the 1 chord. Follow the progression playing roots, you will want to play the 1 chord for 4 bars; that's a two draw. The 4 chord for 2 bars; that's the one blow. The 1 chord again for 2 bars; that's a two draw. The 5 chord which is one draw, 4 chord which is one blow, and the 1 chord which is two draw, and then the 5 chord will turn you around back to the top that's a one draw. So it will sound like this. And that's back to the top."