At which point?
You get into Fastboot Mode on the phone?
But fastboot devices on the computer doesn’t find it (returns an empty line) or gives an error message (which one exactly)?
Make sure to try the fastboot command from the current Android SDK Platform Tools.
On Windows make sure the correct driver is installed, here’s how … [HOWTO] Fairphone 3/3+ stuck in fastboot/bootloader mode with the Easy Installer on Windows