When you set up a bass guitar chord progression on a loop and just improvise, several things have to happen. You have to know the 'correct' notes to play, where they are on the bass neck, what rhythms to use, how to make it sound musical, and what techniques to use.
All that is discussed in this video lesson. Make sure to scroll down for the backing tracks and scale shape downloads.
Bass Chord Progression
The key is E Natural Minor and I used Em, D, and C chords from the harmonised scale. Those can be played using classical finger style technique.
What Scales Do I Use?
You won't go wrong using the following scales over this chord progression.

E Natural Minor

E Minor Pentatonic

E Minor Blues Scale
There are loads more lessons on scales and theory on this site.
Bass Chord Backing Track
Download and use the percussion track and the bass chords (if you can't loop them yourself obviously).
Here is the percussion...
And the bass chords:
Bass Scales FREE PDF Download
Download the PDF below and memorise them all. If you got something from the lesson please subscribe to my weekly newsletter (where I'll send you nothing but more stuff to help you get better!). Please consider making a donation to keep the site going if you want to see more free weekly lessons like this one.
Next course how to play through jazz changes
Thanks for the suggestion – it’s a very good one! All the best. Dan.