Two things that would go a long way toward increasing historical accuracy (to the point that I, with a concentration in the late Anglo-Saxon era, can ask intelligently about the plausibility of things in a story set in 15th-cent. Bohemia): consciousness that medieval human nature was much like modern human nature, with the same capability for sublimity and grossness, although with different priorities and ideals (and you ought to have a working knowledge of what those were); and a knowledge of what kind of question to ask.
Knowledge of the former point keeps you from saying “but everybody was unwashed and illiterate and the Church oppressed women!” which if you’re going to attempt a story about real people is possibly the most damaging mindset you could have. Do better (it’s not hard!) than my anti-theistic Brit. Lit. classmate who, on the day we discussed the Wife of Bath’s Tale, went off about how in “ancient times” (never mind that Chaucer was practically yesterday) women weren’t valued outside of marriage and if your first husband died, too bad, you’d better get another one. Now the Wife of Bath is not a role model in every point of behaviour, but an important fact about her situation is that she sought out at least four of her five marriages (and she says she’s not opposed to number six!) despite that after her first husband died people told her it was fine to live off her inheritance and be a sedate widow, according to custom. She’s also very outspoken, and when my classmate (who claims he is a feminist, because that riles up Christians too) talks over her that way — at that point you’re not even reading anything into the text, you’re just not reading the text. Let the woman speak for herself! That’s principle one: let the people of the time and place you’re writing about speak for themselves.
That means reading what they wrote as often as possible, but also setting aside your preconceptions of what they were like before coming to the work. The essentials of human nature don’t change over time — which is part of what makes things made in, or written about, past times so wonderful in awakening us to a knowledge of shared humanity and something almost like the massive continuity of ducks. However, lots of the fluff around the core changes wildly, to switch metaphors, and understanding, for example, that lots of the customs we consider so important would be as ridiculous to our great-great-great-grandparents as we consider theirs, is also valuable.
For point two, you don’t have to know whether hams were wrapped in waxed paper in 15th-cent. Bohemia; you only have to ask. And then go find out. It’s already a much better question than “what food did medieval people eat”.
Here’s a thing that will help you a lot in researching anything: don’t ask about “X in the Middle Ages”. The Middle Ages, although definitions differ, encompass about a thousand years, and if we’re talking only about Europe, half a hemisphere. That’s huge, to start with. Marriage customs in eighth-century England are not like those in eleventh-century Italy (the geographic location; it wouldn’t be all one country for several hundred years), or in fourteenth-century Spain, or sixteenth-century Russia, or. . . you get the idea. You can Google “medieval marriage customs” but I can guarantee you will get inaccurate sources. (For example, every. single. listing of ‘medieval wedding dress’ — they’re white, and princess-seamed, and all kinds of other inaccuracies. Part of that is the market they’re catering to doesn’t know or care about accuracy, though.) Instead, be specific about time and place. “Marriage customs in fifteenth-century Bohemia” is a lot better right from the start. There’s actually hope that someone’s written a book, maybe even one called Marriage in Bohemia in the Fifteenth Century, and that the authour consulted the proper primary sources about it. Library databases are a good place to look (Jstor especially); Google Scholar is not half bad; I highly recommend Academia.edu, even though they e-mail every week asking you to upgrade (which involves money; the basic account does not). Re-enactors’ blogs are also good and sometimes provide lists for further reading.
See, I don’t have to have crazy in-depth knowledge of everything under the sun to be good at this. A lot of the important things boil down to two questions: “How do we know that X?” (for x=anything, like ‘waxed paper was used to wrap meat in 15th-century Bohemia’; I don’t have to know, just notice the detail and question it) and “Are these sources reliable?” The latter is a bit trickier; a statue from the time and place is a reliable source for thirteenth-century Norse carving, but not necessarily for telling what thirteenth-century Norsemen wore on their feet. The sculptor may have depicted men with “what looks like” boots (you’ll hear variations on that phrase all the time), but was he accurately representing what he saw, or were his priorities in the art something other than strict realism? With regard to the style of sculpture, that’s a primary source; with regard to shoe styles, a secondary. A paper written about sculpture in the time and place, using pictures and analysis of the statue, is a secondary source; on footwear, using same, a tertiary. The reliability of the papers varies depending on how well they interpret their evidence and how critically they engage with conflicting evidence from comparable sources, and opinions of other writers. A random blog post on “Medieval Men’s Shoes” (way too broad a focus, you can see already), which references a paper which cites the statue but badly misinterpret’s the artist’s intention (maybe archaelogical evidence proves something the opposite of what we get from the carving), and lumps 13th-cent. Norway in with 15th-century France because they’re kind of close, right? is a Very Bad Source, and also quaternary if we’re still counting.
And so on.
N. B.: I am not actually working on anything of my own set in 15th-century Bohemia; it was a convenient example because I was beta-reading someone’s short story set then and there. And it’s not actually all that bad. I had a lot of related rants waiting to come out.