Is there any guidance on how to install it on another phone?
Yes, use the exact same guide you quoted that from and follow the instructions.
Officially supported operating systems for the web install method:
Google Android (stock Pixel OS) and other certified Android variants
That means you can use either a Google Pixel phone with the stock OS to connect it the Google Pixel device that you want to flash GrapheneOS on to via the browser app using the web installer method. Or use a certified variant to flash from that phones web browser app to the Google Pixel device you want to install GrapheneOS on to.