Something I may actually be able to help with! I'm a bass player learning guitar, so putting together bass lines comes naturally to me.
What I do, I'm not saying it's the right way or the only way, but what I do is always make sure on 1 to be on the root note and on 3 to either be on the root or the fifth of the chord. In G, that means the first beat of the measure (in 4/4 time, which most rock, country, and pop songs are) you will start with a G. On the third beat of the measure (same assumption) you'll either be playing a G or a D.
Given that you know where you are with a G or a D, then all you have to do to make the bass line "walk" is to play some fill notes (also called passing notes) between the two notes.
So let's say you're playing in a chord progression that goes G to C to G to D, which is a real common one.
The notes in the G chord are G, B, D (1, 3, 5). The notes in the C chord are C, E, G. The notes in the D chord are D, F#, A.
If you're on a G and walking to C, you can use any common notes in scale and concentrate on common notes in the chord. So in a bass line, I may go from G (root note on first beat - hold it a little extra or hit it a couple of times) to A (second beat continue shuffle rhythm from first beat) to D (third beat - note it's the fifth of the G chord so hit it hard) to C# (passing note) to C (First beat of next measure). That'll be a little pop music going up to D to come down to C and will give you some tension on C# because it's not in C's or G's chord. But you just play it for a moment passing between D and C. Then walking back down from C to G, I'd go from C to B to A to G. Then G to B to C to D. I'm talking notes here, not chords. The G note, not the G chord. Just keep "walking" the scale, mix up your rhythm with a bit of a shuffle, and be sure to be on the root on the first beat (1) and the root or the fifth on the third beat (3). The notes between are just in a flurry of notes and if you mess up call it tension and say it was jazz. Bass players call this "playing in the box" and is an extremely popular technique in country, blues, folk, and rock. Folk songs use a lot of sevenths in their walking lines. Blues uses a lot of flatted thirds and flatted sevenths in their walking lines.
I hope that served to help and not to confuse.
- Zurf
Granted B chord amnesty by King of the Mutants (Long live the king).
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