I am trying to learn theory. How can I create a bass line that has a I-VI-V-Vb progression?

Answer 1

on a very simplistic level, decide what key you are in. For example, if you are in the key of C, that is scale degree #1. Scale degree #4 in the key of C is F; scale degree #5 in the degree of C is G, then a Vb is g flat.

so there you have C, F, G, Gb. Then you have to fill in the harmonics above the bass line. Hope this helps. Kind of hard to explain just writing it down!

Answer 2

Amy J is right; finding your fundamental tone of the key you wish to use is the first step. Then count up the diatonic scale, mindful of the frets you must skip, and play the IV, the V, and the flatted V.

That run there seems you have a Gmaj7 thrown in the mix; a most beautiful chord or simply a D chord.

Regardless, your best option is visiting a music store and buy a booklet designed for Bass guitarists having fundamental music theory for you.

A good bass guitar is the cornerstone for any competent band; I love them, but I only played a bass one time.