In order to improvise on bass we simply need to know all of the scales that “go” with all of the chords. That comes from 1.) Memorization 2.) Practice. First we need to memorize all the major, minor, and altered scales. Then we need to combine the notes in these scales to create chords, and memorize which chords come from which scales. Then we need to spend the rest of our lives (or more, if possible) playing the scales that “go” with these chords, figuring out licks, and learning other people’s solos (figuring out how they approached playing over these chords.) Simply put, if we memorize the C major scale, and use the major scale to make chords (like a C major chord, for example,) we learn that when we see a C major chord, we can play the C major scale. That’s the simplest possible example, but the concept remains the same for the duration of our study.
When I teach lessons on improvisation, I will require you to memorize all of the keys (scales) and all of the chords in these scales. Then I will have you apply this knowledge by writing solos over jazz chord changes, incorporating all of this knowledge into creating actual musical lines.
Jazz improvisation on bass starts with the basic technical knowledge of chords and scales. Once the chords and scales have been committed more or less to memory, one begins the process of writing ones own solos over chord changes, and transcribing solos that other players have played over these solos. Of course, the goal is to learn all of this stuff so well that you forget about it, and you can use the musical knowledge you have gained to actually create and communicate something deeply personal that has little to do with technical acumen.