Now I come to the contradictory part and my final rule for writing historical fiction.
You have to know the details; you should know them so well that you could walk down a street in your time-period and fit right in. You have to show the customs, which means you have to know them. You have to be able to tell your reader the details they need to know and use them to give the reader a feel that they're there. You have to leave out the ones that don't.
Probably 75% of what you've learned you should leave out. But how do you know you can leave it out if you don't learn it in the first place? It's rather like fantasy worldbuilding, in which most of the worldbuilding should give a feel that there is more to the world than is ever being told--because there is. There had better be more to our historical world than we're telling the reader, or the reader will sense it.
My editor for Freedom's Sword had to slap me not long ago for using the correct names of horse armor. Why was I doing it? Showing off really. "See what I know!" It added nothing to the story or the sense of place. It just showed that I'd done a lot of research. Well, whoop-de-f*ing-doo. That is my job. It's nothing to show off about.
The reader wants to smell the shit in the street as your character rides through, if shit there is. Hear the creak of the dray. Smell the hay. Feel the scrape of the armor as he prepares for battle. Hear the vendor in the street crying his wares. Taste the food in his mouth at the banquet or at the campfire on the road.
What we have to try to do is leave out the details that don't do any of that. And put in all of the ones that do.
Please check out my historical novel, Freedom's Sword, available on Amazon and Smashwords.