If you plan to relock the bootloader after the installlation
(not mandatoty, only available with Official version, why relocking ?)
Most important thing to take care about is the “android Security Update Patch Level” in the already installed Operating System on your device before you start the install process.
It must be OLDER than the “android Security Update Patch Level” included in the /e/OS build you want to install, otherwise you risk to change your device into Brick.
…
other things you must take care about
Your Stock OS Android version, must be the latest available relative to the /e/OS build you want to install → not higher !
Remove all Gogol accounts, screen lock or SIM lock from the device.
Use a USB-A to USB-C cable on a rear USB port (no hub even internal if possible)