Play the open D string with your right hand index finger, then playa double-stop firmly on the D and G strings. Use enough force to create a sustain sound. It may take a while to get an even sound.
The double stop used in Lick 90 and 91 are called power chords (sometimes called a 5th).
Lick 92 and 93 use the same techniques as the previous two licks but replace the power chords with an octave.
The following three licks break the double stops into two consecutive hammer-ons.
Play the above licks again but this time use the right hand thumb for the slapping technique as in Lick 84 and 85.
The next two licks illustrate how you can change a bass line to make it "bigger" and stronger by using double-stops and thumb slapping.
Lick 97 is the basic lick.
Lick 98 uses double stops (power chords).