Don’t edit. Again, might sound counter intuitive but to get the juices flowing, just write and don’t worry too much even about grammar and spelling. It is the easiest trap in the world to revise and edit the page or paragraph you’ve just written ( third go at this one now), but getting the framework down is the important bit. There’s plenty of time for the delights of revision later.
Don’t be too rigid on what you meant to write. Already I had to ditch numerous facts and theories in my research, and it is so tempting to force evidence to fit a neat hypothesis, but in a work like this, the truth is the important bit.
The same rule applies to structure. I may find my original plan on presenting the history reads better in some parts than others, in a different tone than intended, or some historical figures turn out to be more useful than others. Which leads me onto something I have to remind myself…
I’m writing for you, not me. I have to be comfortable and enjoy what I’m doing, but if the way I present it doesn’t interest you, then its pretty pointless going for publication. Some facts and stories I may find dull, or have heard a hundred times; doesn’t mean you won’t love them.
Try and include something about Illuminati and unicorns. People always want Illumanati and unicorns.