This is the very first bass lesson I posted and I'm pleased to say, it has helped hundreds of thousands of beginners get a grasp on music theory and start their journey with playing bass guitar.
Thinking in terms of shapes will help with your improvisational skills. Try this lesson and then try to apply it to the chart lesson.
Here's an exercise on playing over different rhythms. Practicing over different rhythms will improve your chops.
Bass harmonics is another technique you
can use to create music. Using harmonics is also a good way to tune your bass​.​
Step up your bass skills and incorporate
ghost notes ​into your playing. Ghost notes
add a percussiveness to the groove.​
Chords... sometimes overlooked by bassists, they are extremely important
​for a bassist to know. Knowing how chords are constructed will help you create better bass lines. 
In this lesson I show how to improvise a bass line to the BTO song Takin' Care Of Business.
Walking a bass line is essential if you're a bassist. Here's a lesson for beginners, intermediate, and advanced players.