Obviously, you care about the quality of your work, or you would have done all your research in a couple of YA books and Hollywood movies, and not bothered stopping in here. You want to write the best kind of history lesson there is, a time-trip to the hearts and minds of people as they lived in earlier ages. Consider these two essays as innoculations against making the most common and, to us readers, most irritating mistakes of anachronism -- letting in attitudes, events, habits, words, and so forth, that don't belong in the period in which you are writing.
The first thing you must do is never assume that anything we do in this day and age carries backwards -- simple, ordinary, everyday things especially.
Even if you _know_ people have done something "for hundreds of years" you may find it is really only a late Victorian innovation, like houseplants. Before then, like horses, plants belonged outside or in special dwellings of their own. Table knives used to be specially designed to be eaten from in the early 1800s, rather than using a fork. For most of history in most of the world, in the upper classes men and women have dined apart, severely limiting scenes of clever dinner conversation between hero and heroine to after, say, 1760 in Europe. Peasants always pile everyone in one place, but peasant behavior is not the pageantry that sells historical novels.
Equally, until about the same time, when the earliest springs were invented for carriages, riding in them was no more comfortable than rattling over rough asphalt in a kid's red wagon. The carriages of state were ornamental displays driven slowly over streets prepared in advance. Your earlier-period characters do not enjoy recreational drives in the country, especially not with the axle-deep rutted roads. The "carriage trade" as it will later be known, goes on horse-back. EVERYONE of the upper classes rides, even the most frilly feminine on an old placid palfrey led by a groom, because the only alternative is walking if you are ever to leave your manor grounds. Invalids who must be moved are carried in horse-litters, enclosed bed-boxes with one horse before and one aft. In town, even through the Regency Romance period, many people prefer to take carry-chairs, and chair-porters are a specific sub-group of the working class found on the streets, stout, strong, and dangerous when irritated.
These are the major points of existence: eating, drinking, travelling, socializing. Yet by assuming too much -- and thereby allowing yourself to slide out with less of what you think of as "homework" -- you load what might have been a superb novel with killing anachronism.
Researching a novel is not homework, to be despised and avoided. As many of the best will tell you, it is by reading and other research that they find their plots and characters. With a decent amount of research, most of your incidents will be handed to you. You will more often have to cut back from all the fascinating things you could show to only seven or eight big events.
Figure your research will take twenty to fifty books.
The bad part of research is spending a week on a book, only to find that it was not much help.
With the bibliographies you will find here, you will be guided away from the fuzzballs and into the good stuff. You will find which are easy reading, which have lots of pictures, and which are terribly abstruse. You also will know which easy-reading ones are misleading, and which heavy texts are really worth the trouble to hack through. Figure the average non-fiction book should take you as long to read as a 100,000 word historical novel, balancing slim, picture-rich books against the occasional text-rich specialist volume. Many books cover more time than you need, and you need only read part of them.
If you really are interested in the period you have targeted for your story, the research will be fun. If it is a pain and a drag, maybe that is a sign that you have picked the wrong period. Writing an historical novel is mentally going to live in that era and place for the time it takes you to finish the manuscript. If you do not like the era, you may not get the novel done. Better to save time before it is wasted, and look around for a more congenial spot during the research phase.
The big things to nail down first are housing, food and drink with the attached mealtimes and manners, clothing, transportation, and history. You cannot blithely ignore wars, especially ones which roll over this district. You have to know who the ruler is, young or old, pacific or belligerent, charming or monstrous. You need the anthropology or sociology of the period: who has control over the characters' lives and to what extent, how and when people marry, with what criteria, how one advances in life.
Then there is the little stuff, which may not seem importatnt, but especially with a female protagonist the oft-ignored life of women can zap you.
Of course, all matters concerning female organs are despised and erased from fiction, except sex and perhaps pregnancy, and no-one, even women writing for women, ever thinks about them. Yet you had better have the answers in the back of your mind if your heroine is held captive in a tower cell for more than a few weeks. You may find just the plot device you need here, too, if a female character simply can't crawl out of bed for the cramps and vomiting, let alone attend the ball whence she was supposed to elope.
As well, throughout most of history, women of all classes have spun fibres into thread or yarn, and often done the main, simple weaving of household and body linens. Only in the Industrial age, again after about 1760, were ladies relegated to nothing more strenuous than embroidery and tatting. Up into the 1800s, all but the wealthiest men had their shirts sewn at home by their womenfolk. You need some books on these skills. For the masculine side, physical combat to protect self, women, children, king, and country was considered both a perquisite and a responsibility not to be shirked, so that you need to look into the construction and use of personal arms in any period you are writing. The art of fencing covers a very tiny portion of the history of the sword.
As a fiction writer, you know dialog is characterization. It also tags period, place, and class. The challenge is to give the character the speech for the correct time and place, at least not for the wrong era, which should be a matter of priority attention any time you are doing revision or review.
How did people say, "He's uptight," before 1965? One of us just about shredded a thesaurus trying to find a synonym that was close enough. Do you realise that the tabloid jargon "royals" has only arisen since the early 1980's? Your characters cannot use it even in the 1970's; it would be simply not understood, or only understood dimly, rather than making them sound like gossip fans. Even an illiterate footpad will not refer to a Renaissance king and his court as anything but "royalty" or "the royal court." Those were the only words available: "royals" is an anachronism.
The trap here is that we are often unaware how what was recently slang has shouldered its way right into acceptable English. We think in these words. If we are too inattentive in research or hasty in revision, we will leave our characters thinking in them, too.
First, you need a good grasp of Standard English, which very few people speak as a native language. It's not slangy TV English. Read lots of non-slangy nonfiction books from the 1920's and 1930's to renew your acquantance with it. Then, when you write, write dialog in Standard English, not the colloquial English you use with your family and friends. At first, it feels and sounds fussy, like an eighth-grade English teacher constantly correcting you -- guess what he or she was trying to teach the class to speak. Very quickly, though, it becomes part of characterization, that back then people of the better class talked that way. You will find it much like switching your internal editor over to avoiding contractions in your text, though you use them all the time in speech and letters.
Secondly, you need a good book of slang, and possibly a second one of catch-phrases. Old ones from the second-hand store are often best, because ten or more years ago the authors were aware of the novelty of words a current writer would not think to examine. You should read these cover to cover just to remind yourself what phrases these include. It will be like when you trained yourself to get warning bells at cliches in your writing. Examine every phrase. Something as ordinary as "she checked out the other rooms" would not have been written before 1968. In 1964, the phrase was "she checked the other rooms." "She investigated (or looked through, or examined, or inspected) the other rooms," would be best before WWII. Anachronisms in text jar the feeling of immersion in the period, too.
Other anachronisms have to do with technology. People used to say, "This happens at an inconvenient (or convenient or disastrous or opportune) time." References to "timing," good or bad, are part of 20th century, stop-watch, techno-talk. Being "all fired up" or "having a head of steam" are 19th century, Age of Steam phrases. Equally, no-one "goes off half-cocked" before firearms are common, nor do they sell or buy "lock, stock, and barrel," (complete makings of a rifle), "shoot their wad," (use an opportunity ineffectually, as when you fire a muzzle-loader with only the wad in, having forgotten the ball), nor "hang fire" (waiting for something anxiously, as when incomplete ignition fails to shoot a round, that might yet go off any second). Find plain English equivalents, or even rethink them in terms of an earlier period.
Using the correct historical colloquialisms instead adds another level of period flavor to your characterizations. If you can either naturally explain the slang, or make it clear in context, your readers really enjoy the verbal ambiance. Think of how Captain Grose's book of slang has taken over dialog in Regency romances!
You do often have the problem that your characters are not speaking English as we know it: Romans speaking Latin, knights speaking Norman French or Middle English, barbarian hordes Gothic or Hunnish, and Pharaohs ancient Egyptian. Standard English will bail you out here, too. Minor phrases in translation or the original will add sufficient atmosphere. For example, your ancient Egyptian does not say, "Hello!" or "How are you?" The greeting is "Life! Health! Strength!" Your centurion says "Vale!" instead of "Goodbye." Greetings and farewells being distinctly not ours is often enough verbal period decoration to remind the reader. Some words or phrases can only be misleading when translated. Some authors use "gown" instead of sari or khiton, giving a false image of a cut and sewn garment, or "knight" for equites, implying levels of armour and chivalry that the Roman class never considered. Keeping these untranslatables will complete the period flavor.
On the other hand, few should attempt to write as if their native dialect were the same as Shakespeare's, or have characters speak forsoothly, unless you have really studied "thee" and "thou" as English structures. It is trickier than most think. Quaker use of "thee" is their own debased dialect which has nothing to do with past English usage. "Thou" is not just an old way of saying "you." Knowing French is a big step forward: "thou/thee" is the exact equivalent of "tu/te," and only replaces "you" where "tu" would replace "vous." It helps to remember that English is as much a descendent of Norman French as of Anglo-Saxon, but which shed its second person singular/informal barely a couple of centuries ago.
Especially, do not "wast and wouldst, 'twas and 'tis" in some places and then use TV English elsewhere. Even the least learned of readers can feel the unpleasant shift of incompatible styles, like a grand opera suddenly turning into reggae. Period-style language has to be one hundred percent consistent. All characters use it, "thou or you" at the right points, or else you only drop in the occasional appropriate period oath or exclamation. Forsoothly text means you never get to step down to your natural language, and is such a pain, we cannot imagine why you would convince yourself that you need it, unless you are writing first-person with a period viewpoint character.