As I mention in the lead post at the Unofficial builds for Google Pixel devices post:
For a new installation, you can read up on the LineageOS install instructions for your device, but as a general rule, first install the boot / recovery image using
fastboot flash boot <iode-...-boot.img>, then boot to Recovery Mode and useadb sideload <iode-...zip>to install the actual image.
But, as @chrisrg notes, with the security patch level issues, it may not work well that way, I am a bit uncertain so would be happy for your feedback if you are willing to try it after I get the build uploaded.