Youtube playing by ear and make a chart. Generally the chart is pretty simple. I just jot down the chord changes for the intro, first verse, chorus, bridge etc. From that point on the chart just looks like......
V2
CH
Solo=V (which means solo over verse chords)
Br (bridge)
CH
CHB (which means chorus broken down)
CH
OT (outro)
Just come up with your own simple system. Oddly enough, I read music and have licenses to some sheet music sites, but it's actually faster just to Youtube it, make a chart and learn by ear these days. I've been doing this for over two decades now. And (don't tell anyone but) there's nothing new on bass for the past decade or so. That is, unless of course, you are trying to learn Victor Wooten, Billy Sheehan, etc. Most bass lines are borrowed from a few guys in "back in the day". Learn some Motown and some Led Zepplin and you're halfway there. James Jamerson and John Paul Jones pretty much taught me how to play the bass (and many others as well whether they admit it or not).