Learn To Walk The Bass
Learning to walk a bass is not as hard as it sounds. I'm not saying it is easy, but it's not too hard to learn a set piece. Learning to improvise the bass lines (as a jazz bassist would do) can be hard, I learnt just by learning a few written pieces and transcribing a few Joe Pass pieces, you soon get the idea. Kinda like chromatic arpeggio playing!
Start by learning the chord shapes for A-7, also written Am7, (TAB: 5 x 5 5 5 5) and D9 (TAB: x 5 4 5 5 5), both around the 5th fret.
Get the bass going
Check out the first example and see how the bass is playing constant 4's with your thumb and then three notes of the chord are plucked using your fingers on the and of 1. Listen to the example, try to imitate it and try to get the bass part consistent. It is important that the bass is the strongest part. Make sure that even when you change chord the bass stays in time and consistent. Remember that the bass guitar is part of the rhythm section, so make sure you get your rhythm tight and in the pocket.
And add another chord...
When you have that down and can play it easily then move onto the next example. As you can see the chord now comes on beat 3 as well as the and of 1. This is a very common pattern and sounds great. Listen to the example, walking intro part 2, and again try to replicate it, keeping the bass tight and even. You might use one or the other or combine them, experiment from yourself.
Practice these examples until you feel happy with the technique. The have a look at my arrangement of Fall Off Trees (use your title imagination) to put it into practice....