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 FairphoneOS “android Security Patch Level” already installed on your device before you start the install process.
It must be OLDER than the “android Security Patch Level” included in the /e/ build you want to install, otherwise you risk to change your device into Brick.