I followed those at first but it didn’t work out tbh. The phone somehow never booted to recovery when I followed the procedures exactly. It worked better with this guide
The procedure described for the old, unofficial LineageOS ROM should still work.
You can find it over here: [ROM][UNOFFICIAL] LineageOS 16.0 for FP3
There’s also a mistake in the official guide, as it says in “5. Note”:
if fastboot fails to flash to
boot__a
, you must flash toboot_a
Maybe the procedure text needs an overhaul in LOS wiki.