Thanks for the links, followed the LineageOS guide on how to install adb and fastboot and also watched a couple of videos to be sure, so now hopefully I have those tools at the ready.
I believe the next step is to enable OEM Unlock on my phone. I haven’t been able to do this in the phone settings so I’ll have to do it through the command line, as per this guide. Next I’ll have to install TWRP (version 3.2.3.0, not the newer versions, yes?) making sure to
not restart in the system after flashing the image
I’m assuming that means holding the Volume Up button when I fastboot reboot, as per step 6.b?