It will be necessary to know which of the three bass strings within each basic chord contains the key note. First ensure you know all the notes on the bass strings in the first four frets of the guitar. Study the adjacent diagram. Some common basic chords are shown next. The root bass note within each chord is highlighted with a box
It is important to note that the root bass note will be same for all types of chords. For example A major, A minor, A ninth etc share the same root note (A).
The other chords in the key of G, C and D are introduced in the following example. The standard alternating bass line for the C chord alternates between the fifth and fourth strings. The standard alternating bass line for the D chord alternates between the fourth and third strings. In the following example the left hand changes between the chord shapes in the normal fashion as the right hand thumb picks the selected bass notes.