Bottom line: The chords/riffs come first, the vocal melody comes next, and the lyrics come last.  This is a subtle difference from writing a pop song (where the vocal melody should come first).

Start with a riff.  What’s a riff?  A riff is usually performed on the electric guitar, acoustic guitar, or less commonly, the bass guitar.  The riff is a melody played out as moving chords or moving notes within chord positions.  So, just like writing a pop song, it all starts with a memorable melody.  Your riff may support just the verse while a more basic chord progression supports the chorus or vice-versa.

Secondly, consider the vocal melody apart from the lyrics.  Put a vocal melody on top of your riff and chord parts.  Don’t use words.  Just sing “na na” until you have a melody that you can use for the verses and one for the choruses.  Don’t mirror the riff!  The vocal melody should be different melodically and rhythmically from your guitar/riff parts.  They should compliment each other - not compete with each other.

Finally, after you have your riff/chord structure and a vocal melody to go with it, it’s time to write your lyrics.  The same rules apply here.  Use metaphor.  However, if you can sneak in a common saying or phrase (colloquialism), you will significantly improve the chances of your listeners connecting with your material.