It's a webapp you open in your browser. Uses WebUSB to speak to your phone throughout the process. It's quite hands off. Enable developer mode on the phone, set a toggle, connect the phone via USB, approve use of the phone in your browser, trigger the download of all images needed, reboot phone, hold two buttons, change a setting in the phones bootloader config using volume and power buttons (only really complicated step for most), then flashing will commence from the web installer. Last step is to change the toggle again to re-enable verified boot again. From now on everything else is an over the air update from GrapheneOS.