Watch the 1-3-5 Blues online guitar lesson by Andrew Ford from Essentials: Walking Bass Lines
The first 2 bars is over the Bb7 chord. It begins with a simple root octave combination to the 6, G, walking down to the 5 in bar 2, to the root and ending with a connecting chromatic note, A that gets us to Ab. Then we have two bars of Ab7, in the first we walk up using a 1-3-6 pattern. The next bar of Ab continues on starting on the 5, Eb to the root and again using A natural chromatically to connect to the next Bb7 chords root. From there we walk down the mixolydian scale, 8 or octave to the b7,6, and 5 which are Bb, Ab, G, and F, and continue that 2 bar phrase to Bb and play a root-5-octave, then we go down a half step to Bm7 and play an octave, root and connecting note, F that leads to the E and a 1-5-1 pattern over the E7 chord in bar 8. Then we have 2 bars of Eb7 that we treat as one phrase starting with the root, Eb, up to the 5, Bb, and then up again to the 9, F. We finish the phrase by descending from the Eb, 5 again, and transition with the last beat of bar 10 with E as a chromatic passing tone. Bars 11 and 12 are a 2-5 pattern where in the 1st bar we start on the root Eb to the 5, Bb, then a passing tone, A that goes smoothly to the Ab in bar 12, then we outline the triad with C and Eb. After a half step connection we play another 1-5 in bar 13, D-A, transition on the 3rd beat with an Ab to G and do a 1-2-3 pattern over the G7 chord. There we shift down to Db minor but we start on the 5, Ab, making it a bit more melodic, to the root Db and then a pivoting motion using two swinging 8th notes on beat 3, Ab and Db. We end Bar 16 with an octave , fifth, and root movement. For the next 4 bars we have a repeating two bar 2-5 progression in the key of Bb, C minor to F7. Bar 17 goes from the root C to the octave and ends on the 5, G which transitions well to the root of the F7 chord, from there we walk down the triad, C to A then start bar 19 on the b7 of C minor, Bb, go to the minor 3rd, Eb and end on the root, C. Our last bar of these 2-5's starts on the root, F, down to the octave and transitions with the major third, A. This piece ends with a turnaround or a progression that leads us back to the top of the song. This turnaround starts on Bb major 7, we outline that with the root, Bb, 5, F, and the major 3rd, D, the next bar walks down by a half step to the root of the next chord Db, then we walk up to the 5 and then a big interval leap down to a chromatic connecting note G which smoothly gets us to the root of our Gb major 7 chord in bar 23, Gb, to the 5 again, which is Db here, chromatically down to C to end the bar and transition to B, the root of bar 24, our 2nd note in bar 24 is F which is the flat 5 of B and appropriate since our chord is B7 flat five, and finally back to the root B.