ACHTUNG! This guide assumes you are running a UNIX-like OS such as Linux, *BSD, or Mac OS X. If you are on Windows, you're unfortunately on your own.

To start, download the latest stable (not LTS!) version of OmniOS (which as of this writing is r151038) from here. You should download the USB installation image, not the ISO.

Next, flash the image onto your USB drive by running cat omnios-{VERSION}.usb-dd > /dev/{USB}, replacing {VERSION} with the release you downloaded and {USB} with the path to your USB Drive. When that finishes, run sync to ensure all data is written to the drive.

Now, you can remove the drive and plug it into your server. Boot it up, get into the boot options (typically by spamming F12), and boot from the USB drive. You should see this: OmniOS LoaderJust press enter here to boot normally.

After the installer starts, you should be at a screen to select your keyboard layout. Set that, then press enter over Find disks, create rpool, and install OmniOSce. Select the disks you want to use for your root pool.

ACHTUNG! I strongly recommend using an SSD for your root pool. If you have additional HDDs for a raid array, we will configure those later. If you have two identical SSDs, you should definitely set them up in a mirror configuration for some added redundancy.
Hostname Selection

Now for the most important step: enter a hostname that isn't more than 8 chars, and make sure it is memorable (I name my boxes after elements ;).