If you struggle to make up basslines in jam sessions, band practice, or when making up your own parts for a session, then you'll want to check this video lesson out.
Download the audio:
This is the chord progression for this lesson:
It is in the key of B Natural Minor.
This is the scale in one octave starting from the 7th fret of the E string.
All the lines from the video come from these notes (in more positions than just the above diagram). The building blocks of basslines are triads, arpeggios, and scales. Here are a couple of really handy patterns you can use:
You can use modes over each individual chord:
Check out more on modes here.
Major and Minor pentatonic scales sound amazing in basslines, fills, and solos. There's a lesson on pentatonic scales here. Be sure to download my free ebook which has all the pentatonic shapes in it as well as lots of other useful stuff.
Don't forget that groove, timing, and feel are (probably) the most important aspects to focus on for a bass player. Work on your timing with these exercises and check out loads of other tips and lessons here. Articulations also play an important part in bringing your basslines to life.
As you can see, a few key elements must come together for you to be able to make up good basslines comfortably. Keep working hard on these things. None of them is tricky on their own. As you learn more and listen more, your ear will improve along with your experience and you'll be creating groovy, catchy basslines in no time!