This first riff has somewhat of a Pantera influence. Dimebag Darrell had a knack for playing chunky sounding riffs with a good amount of dissonance.
We have a lot of Drop D power chords played throughout this riff. Let's examine some of the other things though. First, take a look at the 4th beat of the first measure. That's where we see some sixteenth note slides taking place with octave notes on the Low E string and the D string. I'd recommend using your middle finger for the Low E string note, and your ring finger for the D string note. Use your middle finger to play the Low E while also muting the open A string. Make sure you give the slide notes enough pressure so there is proper sustain.
The next thing I want to look at is the very dissonant bend that takes place towards the beginning of bars 2, 4, 6, and 8 on the G and B strings. So before we actually bend anything, we're playing the 6th fret B string and the 9th fret G string, right? Those two notes played together is a minor 2nd interval. But then when we bend the G string note up a half step, the two notes become unison (the same). These type of bends are an excellent way to produce tension, especially in metal music.
Once you've given Exercise 1 some good practice, head on over to Exercise 2.