Find your voice
One of the key challenges you have as a historical fiction writer is finding a voice that sounds period-appropriate without being obtrusively olde-worlde. You need to be sensitive to the world of your story whilst also appealing to a modern reader. I recommend that you read any text you can get your hands on from the period in which your story is set. Read fiction from the period, of course, but also newspapers, sermons, pamphlets, political speeches, plays. This will help to get your "ear" tuned into the voice and style of your era. To keep your reader immersed in your story and not pulled from the narrative by an incongruous modern word, you can check a dictionary to see if a word would have been around in your chosen era.