Know the genre. Listen to as much heavy metal music as you can, starting with its roots in the late 1960s and progressing through to the present day. Don't just listen; try to understand the structure of the songs, the cues and cadences the bands use, the topics they grapple with, and the way the bands served as a counterpoint to more "progressive" rock music. The more you understand the genre, the better you can put your own stamp on it.
Find a group of like-minded musicians in your area to form the band. Put up fliers advertising for auditions, canvas local music clubs that specialize in harder rock and post announcements in Internet forums frequented by musicians. Be sure to stipulate what kind of instruments you need, as well as a lead singer if you need one.
Come up with a name for your band. It should be something everyone can get behind, while providing a distinct sense of identity.
Practice with your new band. If you're not adept at writing songs of your own yet, practice with cover versions of classic heavy metal songs. Music stores and online sources such as ThreeChords.com sell sheet music for songs. Find some tunes your band enjoys and practice on them as much as you can. Every day would be ideal, but try to find at least a couple of days a week to practice.
Write songs of your own if you can. While you don't need to play your own music, it helps develop a distinct identity as a band. Heavy metal music often entails existential themes while expressing emotions centered around anger, frustration or self-destruction. Focus on them, and on a means of expressing them in a compelling or creative way. Strive to be as original as possible, writing songs that speak to you as an artist rather than just imitating better known bands.
Record demo CDs and hand them out to anyone you think can further your career. This includes agents, club owners, radio station programmers and fans. You can make the recordings at a professional recording studio, or on your own using a microphone and audio recorder. The better sound quality you can get in the recordings, the better your chances of gaining attention. You might also consider posting audio files of a few songs online to increase exposure and help more people to listen to your music.
Work on booking gigs. Contact a talent agency in the area to see about representation and talk to local club owners to find out what the process is for playing there. As you network, you'll come into contact with more and more people who might be able to help.