It always annoyed me, when I was researching, to find a relevant sentence or two in the middle of a large, uninteresting tome. The effect could be that you spend far more time reading than writing books. I don't do that any more. Historical fiction, like modern fiction, tells a story. The story has to be interesting, as well as relevant to the time in which it is set. I write my books straight, letting the plot develop from the characters as well as the setting. Then I go back and edit hard.
Both my universities made me aware that I must not take things for granted. Two simple examples, how did they brush their teeth in 18th century France? Were there rabbits in the Isle of Man when the Vikings landed? It is these small details which can make or break a book. Speech is particularly important. I don't pretend to write, for example, medieval slang, but I do try to eliminate modern jargon. Sometimes this is difficult, because the modern words and concepts seem to fit so easily. Yet it must be done, ruthlessly at times.
These days I write my books, researching as I go along. If I know nothing of the period, I do read about it first, but usually I set stories in eras of which I have some prior knowledge. I write, stopping when I come to a place where people would not act as we do. I have almost a sixth sense when I type words that do not fit. I stop and ask myself - would this have occurred in this fashion? Then I check. For example, light is often a problem in certain scenes, especially at night or in a windowless room. How did they see? You cannot write that the POV person has recognised someone at night, unless you provide a light source or use some other sense, hearing or smell perhaps. People before the 20th century had to light candles or lanterns or torches. There were no matches, they used tinderboxes or other methods. It is these details which add to the authenticity to the tale.
Fortunately, I have great proofreaders who spot my occasional mistakes and cause me to check or make changes. Thank you Oliver, Iona and Gwen. Often I pick up errors for myself, when I am writing the second or third draft but I still miss the odd one. My writing is not perfect and my method certainly would not suit everyone. Happily, I have yet to write that Moses arrived by aeroplane or Napoleon telephoned Ney before Waterloo to wish him luck, but you never know what I might write in the future!