List your band on websites that promote your kind of music to help people who want to book a metal band find you.
Create your own website with downloads of your songs available so that people who might book you will hear your sound.
Learn about venues in your area that book metal bands. Contact them to ask if your metal band can play there. Be courteous and respectful, but always ask how much you will get paid. Try to get a contract saying how much you will be paid before your show.
Make promotional items such as flyers, posters and buttons to promote your band and get your name out. The more people hear of you, the more they will want to book your band.
Find bands that have songs recorded with samples you can hear to help you determine if the band is what you are looking for. You can also go to local record stores and look through local artists' material to find a metal band you might want to perform at your show.
Ask around for advice as to who the best metal bands are in the area. Word of mouth may be the best way to hear of a band that suits your needs. Go to other shows and talk to the band members to find out who they think would be a good fit for your show.
Contact the band through email or phone. You can find this information on a website. Ask how much they charge to play. Make up a contract with the band saying how much you will pay them.
Leave your venue's dates open so that you can book the band for the time and date you want them to play. Make sure when you contact the band that they are available for the time you need them.