Here’s a great video lesson from Scott Devine which should help beginners learn about developing basslines. It gives advice and a method to demystify the process. He acknowledges the difficulty in coming up with exercises geared towards this, and as a beginner, I can corroborate that most of what I’ve seen are speed & technique exercises and then scale & chord exercises.
Right off the bat, he says that the skills which need to be learned include finding or creating a groove, getting a great feel for time (internal time) and then actual knowledge of how to create bass lines & fills.
I love Dan Maines. His groove on early Clutch is the 2nd coming of Geezer Butler to me. His playing tells stories to my ear, unlike the overwhelming majority of rock bass players that I hear.
I remember an interview with Billy Sheehan in which he talked about bass lines and how they’ve changed over the years, from the 50s, 60s and 70s until now. He spoke about how bass lines were melodic and were an integral part of the song, how you could hum a bass line by itself and recognize the song it came from. He also observed how this changed with a lot of modern music.
An example that Sheehan gave was playing part of the bass line for “Stand By Me”. Its instantly identifiable if you’ve ever heard the song. He then played a few bars of repeated 8th notes and asked what song it was. It could have literally been any of dozens. I got his point.