Use the minor scale as the basis for a Metallica-style song. The minor scale is 1-2-b3-4-5-b6-b7-8. In the key of E minor, the basis of many Metallica songs, this would be E-F#-G-A-B-C-D-E.
Play two-note power chords consisting of a root and a fifth. Pick three chords that complement each other. Create a heavily rhythmic riff from them.
Add nuance and a darker sound to your riff by adding an occasional tritone. A tritone is a two-note chord with a flatted fifth, which is the fifth moved down one fret.
Strum power chords with downward strokes for a consistently chunky guitar sound.
Use palm muting in a consistent down stroke rhythm, such as quarter notes or eighth notes. Release the muting quickly every third or fourth note and re-mute. Move between the power chords of your riff while switching between muted and unmuted rhythms.
Break your song into clearly defined sections, with a softer, arpeggiated section to provide contrast to heavier riff-based sections.
Experiment with your power chords, tritones and palm muting until you have a song that lasts 3 to 5 minutes.
Hum or sing nonsense syllables over the chords. Experiment with different melodies until you find one that complements the chord sequence and rhythm.
Pick a song writing subject you feel strongly about. Metallica's songs pack an emotional punch because James Hetfield expresses himself in his lyrics.
Sing lyrics to your melody that complement the brooding nature of the riffs. If you get stuck, check the rhyming dictionary.