OK, I'm going to try to be very practical here. Many readers have emailed us and asked for more books in the Samurai Detective series. The publisher, as you probably know, doesn't want any more because they didn't sell as well as hoped for, even though two of the six books were nominated for an Edgar and one (In Darkness, Death) actually won an Edgar for best YA book of 2005. The publisher really didn't try to sell the books, never advertised them, and on one memorable occasion when we were invited to appear at the Smithsonian bookstore for a presentation/signing, the editor refused to pay for our trip, saying, "We don't promote books that don't sell." And vice-versa, I might add.
Anyway, I am going to try to write a first draft of a new book in the series, and let you watch, so to speak. I'll try to describe the process, and maybe that will show how I write. Remember, that doesn't mean you should necessarily write that way. I think everybody finds his/her own method. And in fact, when my wife and I write non-fiction, she usually writes the first draft.
OK, then. The first thing I do--you're going to think this is boring--is try to find an idea. In this case, I already have some characters, especially Seikei and his foster father Judge Ooka, who have been the central characters in all the previous books.
Talking it over with my co-author, we decided it might be good to have Seikei meet some of the Westerners who were allowed to trade with Japan at this time (early 1700s). After the shoguns banned Christianity and threw most of the foreigners out, the still permitted some trading ships from the Netherlands to dock at Nagasaki, and store their goods on the man-made island of Deshima, in the harbor there. Sometimes Dutch traders came up to Edo and appeared before the shogun.
I think every story begins with "what if?" and this one begins with "What if there was a crime committed against one of the Dutch traders while he was in the shogun's palace?" That's the starting point, and the next step (or steps) is to write down possibilities.