Consider the guitar intro. This will often set the tone for the song-writing process. A slow, creepy intro, like the Foo Fighter's "Pretender" may inspire more thought-provoking lyrics. A Rage Against The Machine intro may conjure up different images all together. Regardless, this is a good exercise for song creation.
Steal from the greats. There will never be another "Back in Black" by AC/DC, but it doesn't hurt to use a riff or two. Metal songs have as much to do with heart as they do with technical ability. Even if the writer grabs an obvious chord from a great song, he can make it his own with some heart.
Decide on the focus of the metal song. Will the song have social ramifications like Living Colour's, "Cult of Personality", or will it be a feel-good song like "Smokin' in The Boy's Room"? In either case, the song writer needs to decide from the onset what type of song he plans to deliver.
Develop the song acoustically first. Metal songs are often created in a very minimalist form first. Layers may be added once the soul of the song is created. A good example would be "Smells Like Teen Spirit" by Nirvana. This is an explosively metal song, but can be stripped down to a very minimal core, because ultimately, it's just great music.