You start with ideas. A period in history, a place, an event plants an idea in your mind. How do you make it lead to a novel?
You start to research. Images form in your mind. You see a captain standing on the deck of a ship, his blond beard flying in the wind*. You know what he’s wearing – coat, boots, hat. You know what he has in his pockets. You live most of your life along with him and all the other characters.
Then, when you know all your characters well enough, you use your imagination to weave a story around them.