No degree in history, though I came pretty close to a minor in history because I took so many history electives.
(Tangent: because I did really well on my AP US History and both AP English tests, I started college with a full semester worth of credits, and also the stupid major I chose--don't go with Communications, kids!--left me some room for electives. So I was able to spend a semester in London taking classes that had nothing to do with my major, one or two of which were history-related and which were AMAZING. It was the best part of my college experience by far, which otherwise was not good. But, yeah, I'm going way off into left field, so let me get back on topic.)
The places and times I write about have always been of interest to me. I've always been interested in the Civil War, and the more I learned about the Civil War, the more I became intrigued by the mind-bogglingly complex social structure that led up to the war and that continued in some form until . . . well, you could argue it's still there in some form. But it feels like the Civil War has been in my blood since I was a child. I've also been interested in the Edwardian age forever, pretty much. I was fascinated by the Titanic. The last few years, I've become more interested in WWI, which is woefully underserved.
As far as research: It depends a bit on the project. If I were to write in the antebellum or Civil War eras, I frankly wouldn't need to do much research, being quite familiar with the era and having written, er, let's see (carry the two, find the square root, multiply by the speed of light)...having written 3 novels and a novella in that era. I'm not quite as comfortable in WWI/the Edwardian era quite yet.
I start broad. I won't start writing unless I have a pretty solid grounding in the important events and social mores and material culture of the time. This means reading pretty widely--no getting around that. I haven't, thus far, just started in on a time period I didn't already know a good amount about, but even after I have a general feel for the era, I will still read more about the more specific topics I will be writing about. I'll keep reading as much as possible about the time period, even as I write, not just to answer specific questions I might have, but to keep my head in the game, so to speak. You never know what details you might come across as you continue your research. They might come in handy when you least expect them to.
I have been lucky enough to visit a lot of the places I've written about. I live in Virginia, so it's Civil War Central around here. I live within about an hour of three major battlefields (Gettysburg, Antietam, Manassas/Bull Run) and another dozen or so slightly-lesser-known ones (Chancellorsville, the Wilderness, Fredericksburg, Monocacy, Winchester, Fisher's Fill, etc.). I've also gotten to visit the places in Georgia that inspired my writing about them, and I got to see the very real places where my very real characters lived and died in England. It isn't necessary, I suppose, but I think it's incredibly important to see the places. I know it isn't always possible for a writer to travel to the place they write about, but for me I don't think I could've written so cogently (I hope) about these places and these people without having visited the sites.
I also would caution against relying too much on television programs, unless they're from a reliable source.
One of my favorite sources is CSPAN American History TV. Obviously, that'll be American history, but they have really interesting programs--talks and lectures from professors and other experts, as well as old documentaries and news reels and oral histories. It's all available online. I often put it on in the background while I'm doing things like cleaning or sewing.
Also, museums. Museums are amazing resources. It's great if you can go to a museum related to your time/place, but many museums also have great online resources. American history again, but the National Museum of Civil War Medicine has great online content. And with the quarantine, a lot of museums are hosting talks. The Museum of Civil War Medicine has had a great series of talks. A recent one was a discussion with a curator from the WWI museum about how medicine progressed from 1865 to 1918. It was fascinating--two experts who know their stuff having a blast talking about the stuff they love and spouting all kinds of knowledge.
The point is, there's a lot out there right now. Keep your ears and eyes open. Take in as much as you can, always. It's a never-ending journey.
Something else that I think I've noted here before: one of my strategies is to find a real person who is my key to a time period. Something about this person corresponds to the character I want to write about, and I use this person as a starting point. For instance, I wanted to write about an actress in Philadelphia c. 1830, so I read all about Charlotte Cushman. I wanted to write about a young woman marrying into a slave-holding family, so I read all about Fanny Kemble. From there, I follow the research threads to fill out the world I wanted to create.