To form a scale for improvisation over a minor 7th chord, simply flatten the 3rd and 7th note of the major scale (that starts on the root note of the chord).
This produces the so called 'Dorian mode'.
Like the Mixolydian mode, the Dorian mode is a displaced major scale with an different tonic note. The major scale related to the Dorian mode has its tonic note one whole tone lower.
The C Dorian mode is derived from the Bb major scale.
There is only 1 note different between the three modes. The D Dorian mode contains a B, while the the C Mixolydian and G Dorian modes have instead a Bb.
These are the critical tones for improvisation over this song.
Starting a phrase on beat 1 of a bar is OK initially, but it can become rather predictable to listen to.
Good improvisers start phrases at any point in the bar. This creates great interest and variety in their solos.
Check out for example the transcribed solos of Charlie Parker in the 'Charlie Parker Omnibook'. His phrasing is an outstanding example of what to aim for.
There are eight different starting points in a bar,
the four down beats (1 2 3 4) and
the four offbeats (+ + + +).
Practice starting a phrase on each of these eight points. Concentrate on one point at a time.
Rhythm Patterns 31 - 35 start either on 1 + or on 2.
Try also to do some free improvisation starting on these points.
Start practice on the Dorian mode in all keys.
First priority are the major scales in A, D, G, C, F and Bb.
Then add the others : Eb, Ab, Db, Gb, B, E.
