Sometimes an event or mood will inspire me to write lyrics
Sometimes a song by another band will inspire me to write lyrics
Sometimes a riff i've made will inspire me to write lyrics
Sometimes an event or mood will inspire me to make riffs
Sometimes a song by another band will inspire me to make riffs
Sometimes lyrics i've written will inspire me to make riffs.
Some people do work differently and always use their lyrics as a starting point for a song or vice versa, but for me the most organice way i go about the creative process is as above.