A good guide for creating a basic walking bass line is by using approach tones. Generally, when we look at a chord progression we want to place the roots/chord tones on beats one and three and approach tones to the roots/chord tones on beats 2 and 4.

Check out the progression below. I have a chart that reads 2 chords per bar in the key of C.

It's Cmaj7 A-7 D-7 G7

An approach tone is by it's very definition a note that "approaches" the next note. In the example below we can see that we used notes on beats 2 and 4 that approached the chord change by a half-step below. Take a listen.