| In its most basic form, walking bass consists of two elements: chord tones, and approach tones. In a nutshell, you should play chord tones on the first three beats of each measure, (assuming that you're playing in 4/4 time) and play an approach tone on the fourth beat.
Approach tones are notes that facilitate a smooth transition between chords, they fall into three categories: scale tones, chromatic passing tones, and dominant approach tones. Let's say that you're playing a tune that goes from a D minor 7 chord to a G Major 7 chord. In the first measure, you might play D F and A for the first 3 beats. Since you're probably going to play the root of the next chord (G) on the downbeat of the second measure, you're going to need a smooth sounding approach tone to get from the A on beat three to the G on beat one. At this point, you could play F#, which would be a scale tone, Ab, which would be a chromatic passing tone, or D, which would be the dominant approach tone, since D is the fifth of the G chord. Dominants require the most concentration to use, since you have to visualize the fifth of the upcoming chord before you get to it. If you can find it, I'd recommend getting a copy of Ed Friedland's book, Building Walking Bass Lines. It's a very methodical book, and clearly lays out these concepts for those who have not improvised before. | 
