OK, here's my take on this. But, it's also predicated on the idea that you learn good solid physical technique so you don't hurt yourself and you don't cause yourself problems down the road. No wrapping your thumb over the top of the neck, no big bends in your wrists, efficient and logical fingering, and you learn to play with your fingers and a pick.
Also, learn to stand before you try to walk- to walk before you start running, etc. Baby steps!!
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THEORY PROGRESSION
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Theory can seem like a quagmire to those who are starting out, and it's often difficult to know just how important a particular aspect of it is. I will say that learning how chords are built from scales is the most important aspect of theory. It is far more useful to understand chord construction than to memorize all those "Scales A and B go with chord X" formulas.
I'd say the logical progression learning music theory is this:
1.) Learn the major scale, and how it's constructed
2.) Learn how basic chords are built from the major scale- e.g Major is 1,3,5, minor is 1,b3, 5, etc.
3.) Learn how to harmonize the notes of any diatonic major scale by building chords / stacking thirds.
4.) Learn arppegios/chord tones
5.) Learn to look at common chord progressions as "numerals" (eg, I-IV-V ect) to understand how the chords relate to the song's key.
6.) Learn the Natural Minor scale (a/k/a Aeolian mode) and the dominant scale (a/k/a Mixolydian); And learn how these relate to the major scale (i.e.; its the V and vi mode)
7.) Understand how other 4 modes of the major scale are derived (less important to memorize these other modes at first)
8.) Dive back into modes for more detailed ideas about what "goes" with what chord.
Bass playing is basically a matter of knowing what to play over various chords. It may seem daunting at first, but my practical experience (bass in pop/rock) has been that I mostly use Major, Minor, and Dominant 7 related bassline patterns, usually based on chord tones and pentatonics. Even if you're playing some guitar oriented riff-rock, each riff is going to imply a chord of some kind.
"BUT HOW DO I APPLY THIS THEORY TO MY PLAYING?"
85%+ of the time, you will be going from root note to root note as the chords change. The trick is learning how to do it with a groove and feel that is stylistically appropriate to the song. The best way to reach stylistic understanding is to learn songs you like and pick them apart to see how the bassline relates to the chords. I cannot emphasize this idea enough: The answer to this common question is to LEARN AND ANALYZE BASS LINES BY THE MASTERS. Once you undertand what Jamerson (for example) did with a particular set of changes, these ideas become added to your tool set, to use, change, blend and create your own voice.
