I think the Gm-section of Kult divinity lost is pretty great, as it explains you how to build a mind-map for your metaplot and how to use PBTA mechanics to make the plot move in the background.
Now what I practically do is outiline the main theme and the reccuring NPC, then rely a lot on what the PC want to do and how they act to see how it develops. There is no point in writing long texts nor planning too much in advance when the PC will break everything by killing the wrong person