how to set up nginx server blocks (virtual hosts)?

I have a Ubuntu LAMP server with nginx as the front end proxy to access site1.mine.com. I need to set up two more accounts/virtual hosts on the same server (site1a.mine.com, site1b.mine.com) pointing to the same IP. I have these subdomain names created and pointing to the DigitalOcean server IP. However, I can’t use the ‘How To Set Up Apache Virtual Hosts on Ubuntu 12.04 LTS’ tutorial since I am using nginx as the front end proxy. I understand that nginx doesn’t have virtual hosts, but instead uses server blocks. I’m looking for any guidance/tutorial on how to do this.

Thanks.