Heres a relatively straight forward way to start writing a walking bassline. Its by no means the only way to approach walking, but its easy, and it works.
This method uses Ascending Linear and Descending Linear basslines. The main thing you need to know is whether the chords in the song are major or minor. For Ascending Linear lines, it doesnt matter if they are 7th chords or not. You just need to know the defining quality of the chord.
Then you need to figure out the first three notes of the scale that defines the chord. These three notes, plus a passing tone are all you will need to write out for each measure of the song. Since jazz operates in quarter note pulses, in 4/4 time, you only need four notes per measure.
For example, take the chord progression CM7, E flat 7, Dm7, and G7
In order, these chords are Major, Major, minor, Major.
So for the CM7 chord, write out the first 3 notes of a C major scale:
C-D-E
For the E flat 7 write out the first 3 notes of the E flat Major scale:
E flat-F- G
For the Dm7 write out the first 3 notes of a D minor scale
D-E-F
For the G7, write out the first 3 notes of a G major scale
G-A-B
So now you have 3 of the 4 notes you will need per measure. To join the measures together, use a chromatic passing tone. Previously used notes work well but you can use whatever note you want if it sounds good, so the passing tones are in parenthesis:
C D E (D) | E flat F G (E flat) | D E F (F sharp) |
G (one octave lower) G A (B)
Notice there are two Gs together in one measure for the G7. Typically you dont want to use the same note twice, but they are an octave apart, so my subjective music rule says its ok. The B is the passing tone back to the original C
This is called Ascending Linear because you may have noticed all of the notes ascend the scale.
So the name Descending Linear should tell you that the next set of lines will descend the scale, however in this case, you have to be aware of the chord quality since you will be starting with the root and descending toward the 7th and 6th of each respective major chord or a flat 7th and 6th of each respective minor chord.
So the Descending linear line will be:
C B A (D) | E flat D flat C (D flat) | D C B (G flat) | G F E (B)
For a song that has two chords per measure, just use either a root and 3rd or a root and 5th. Its your choice.
So, learn your ascending line down cold, then, learn the descending line cold. Then mix them up. Ascend sometimes and then descend sometimes. Take it all over the place. Just a trick to get you started...