(only available with Official version)
Most important thing to take care about is the Stock OS “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.
- You will find this information about stock OS on the device :
settings --> about phone --> android version 15 --> android security update
- you will find this information about /e/OS build on the install instructions page for you device if your device is supported by /e/
- but also on the release article about the /e/OS version you want to install even if you device have no instructions page :
the title of the article need to match both “/e/OS version” and “android level”,
currently "e-3.2-a15"
Releases · e / os / 
Releases · GitLab
more detailled explications here : (answered) Install /e/ on FP5 with newer Security Patches