Scales are so we know where the notes are on our fretboard. Once you can hold your own with the scales then chord tones, is where you need to be spending your time. Why?
Little side trip into theory. The songwriter places chords in the song where they do two things; 1) move the verse along from rest (I) to tension (IV) to climax (V7) and then a return to rest (I) - so something new, another verse (another thought), another phrase, etc. can take place. And then 2) the chord is placed over/under the melody so the chord's notes will harmonize the melody, aka sound good with the melody. It's this sharing of notes that builds harmony. There is melody, harmony and rhythm. At this point in your journey leave the melody to the solo instruments and lets take care of harmony and rhythm. I'm not going to address rhythm in this paper. Little more on harmony...
Since the songwriter has placed the harmonizing chord at specific spots in the song if we want to sound good with what is happening in the song it makes since to play the notes of that harmonizing chord. So a Cmaj7 chord is coming up in the song we should play some of the notes of the Cmaj7 chord. Yep got to know what notes are in that chord. The Cmaj7 chord has this generic spelling R-3-5-7, i.e the root of the C scale, then the 3rd, 5th and 7th scale degree of that C scale. If a Fmaj7 chord was coming up I would still use the R-3-5-7, but, this time I'd be playing the root, 3, 5 and 7 notes of the F major scale.
Play as much of that spelling as the song will allow. Roots just by themselves will harmonize. A Root-5 would be better and if you have room add the 3 for R-3-5-3, or R-3-5-X. That X could be the walking bass line note that pulls you into the next chord. Long story, my point being scales are for learning how to find the notes, chord tones are what we are asked to play. Notice I'm talking numbers as well as A, B, C's. I'm a pattern guy, my patterns and the major scale box help me with what I do. See the A, B, C and after placing my box, I let the 1, 2, 3's guide my fingers. For example:
Bass Patterns based upon the Major Scale box. Re-read all of this while looking at the follow major scale box.
Link Removed
Major Scale Box.
G~~|---2---|-------|---3---|---4---| 1st string
D~~|---6---|-------|---7---|---8---|
A~~|---3---|---4---|-------|---5---|
E~~|-------|---R---|-------|---2---|4th string
I'm sure you have seen this box with just dots.
Adding the scale degree numbers lets the box become a living breathing visual aid.
Basic Chord spellings
- Major Triad = R-3-5
- Minor Triad = R-b3-5
- Diminished Chord = R-b3-b5
7th Chord spellings
- Maj7 = R-3-5-7
- Minor 7 = R-b3-5-b7
- Dominant 7 = R-3-5-b7
- ½ diminished = R-b3-b5-b7
- Full diminished = R-b3-b5-bb7
See a chord and play it's chord tones. As every key will have three major, three minor and one diminished chord it's a good idea to get your major, minor and diminished bass line chord tones into muscle memory so when you see a chord your fingers just know what will work. Now the song may only give you enough room for the root, or root five - adapt and get as many chord tones into your bass line as needed. Root on 1 and a steady groove from the other chord tones plus something to call attention to the chord change is what we do.
That is the way I do it, that does not make it the only way, that is just one of many ways to play chord tones.
Have fun.