Know your subject matter inside out. The best historical writers are those who really do their research, they delve into the nitty gritty of their subject matter and from there pull characters and stories that have yet to be told. Of course, the fact is not always the most interesting so of course, be creative with your material, put your own spin on historical events but keep your time lines consistent with the fact, as well as births/deaths etc.
Historical novels are steeped in rich and vivid descriptions, remember you are recreating the time period, transporting your reader hundreds of years into the past. You want them to taste the wine, feel the fabrics of the gowns, smell the blood of battle. These novels are based around things that actually happened, the majority of your characters where once living and breathing people just like you and me, as the writer you are giving them new life, re-enacting their stories and giving a possible insight into their thoughts and reasons behind their actions.