Just listen to songs and figure the lines out by ear. You will get much better this way. Take your time and enjoy the time doing so. I use a program called Transcribe! that does a great job of adjusting speed/tempo while keeping the song sound good, and you can easily adjust the pitch to where you need it to match your tuned bass instead of retuning the bass to match the particular song. This is especially important on old songs ('60s-'70s) where the recording/tape speeds were all over the map (sometimes intentionally).

