In bars 9 through 12 of Exercise 2, we have a build-up where just the guitar and bass are prompting the drums to start playing so to speak. The most important part of this section is the use of rests. Make sure the rests are extremely tight sounding, meaning they should be totally silent.
Once the full riff kicks in, we'll start adding in some crunching palm mutes on the open D power chord. Make sure you grind the pick in the strings big time. Make those palm mutes sound brutal!
In bars 17 through 20, we'll be breaking away from the heavy palm mutes and doing something a little different. We'll be playing a few major chords without the 5th. So for example, look at the first chord in bar 17. It's basically a C Major chord but without the 5th degree. So we have a C note (1st degree) on the 3rd fret A string, then another C note on the 5th fret G string, then an E note (3rd degree) on the 5th fret B string.