/u/sunagainstgold has previously explained why nobody should get a PhD in History.

I'm going to give you the answer you want to hear, but first, I'm going to give you the answer you don't want to hear, because it's the actual right answer. DO NOT GET A PH.D. IN HISTORY. I can't emphasize enough how bad of a career decision it is at this point. It was a terrible career decision when I did it ten years ago, and it's exponentially worse now. Getting a Ph.D. at this point is not worth the financial cost or the opportunity cost because there are no jobs. I was one of the lucky few; I survived and got a real history job and if I could, I would absolutely go back in time and do something else, I would in a heartbeat, because it was nothing but blind luck that I made it here. I'm serious, do not get a Ph.D. in history. You will be absolutely screwing yourself over if you do.

If for some reason you choose to ignore that advice (which you shouldn't), the best thing you can do is have as concrete of an idea as possible what field you want to study and what you'd like to spend five plus years of your life researching and writing a dissertation about; the more specific the better. Understand that your major field will be relatively specialized; in your case, probably post-1865 United States or something like that (not an Americanist, I don't know how they divide it exactly). Once you have a good idea of the field you want to go into, you'll want to find senior faculty who teach and research in that field as potential advisors. A good way to do this is looking at who's authoring the most important recent books and authors in your field of interest and then finding out where they work.

After you have some prospective advisors located, take the time to look into the requirements for their universities' graduate programs, both in terms of what you need to apply (typically, transcripts, a statement of purpose explaining your plans, GRE scores, an application fee, a writing sample, etc.) and in terms of what the requirements are for completion of the degree (foreign languages, other exams, etc.) so you know what you're getting yourself into. Then, as you're getting ready to apply, reach out to those professors and express interest in working with them on a Ph.D.; hopefully they'll give you the same advice I did and tell you not to do it, but if they don't, they'll at least let you know whether they're taking new students or not. Also, let me be clear, if you're not applying to top-tier programs, you'll have absolutely no chance on the job market (which doesn't exist anymore anyway). Don't be an idiot like me and get a Ph.D. from Directional Michigan. You'll never find a job if you do that. Is that kind of snobbery fair? Probably not, but it's the way things are.

When you write your applications, be as specific as possible about what you want to do in your research and explain why you and your research are a good fit for the program. Your statement of purpose is not about how you always wanted to be a historian since you were a little kid or whatever, it's about the concrete plans you have to complete a dissertation and graduate. Mention that you've spoken with that professor about what you want to do. This is the stuff admissions committees are going to factor in. Someone who comes in with a clear plan of what they want to do has a much better chance of finishing their Ph.D. and then not finding a job than people who are just drifting aimlessly into grad school because they're good at school, who will probably drop out and also not get a job. Some programs will have interviews and additional hoops to jump through, so again, be aware of what's going to be required as part of the application process. Also, make sure you're going to be fully funded for as long as possible. DO NOT GO TO GRAD SCHOOL IN THE HUMANITIES WITHOUT FUNDING, IT IS FINANCIAL SUICIDE. Also note whether that funding is in the form of teaching assistantships, research assistantships, or fellowships (or some combination of the above), and look at what they have as far as supplemental funding for research/conference travel, etc., because you'll need that too.

But really, you shouldn't do any of that. I'm sorry if this sounds mean and I'm coming across as a bitter, cynical old man who wants to ruin everybody's dreams, but it's the truth that undergrads need to hear right now. The future you imagine, of getting a Ph.D., getting a tenure-track job, and growing old in your office with your pipe and elbow-patched jacket does not exist anymore. Getting a Ph.D. in the humanities is an objectively terrible career decision in 2021 and absolutely nobody should do it.

The way I think it works is that you look at history graduate programs for the professors' interests and find faculty who research what you're interested in (you can sometimes find them by looking at history journals for recent articles on research you're interested in, then look at the universities that they work at). Your dissertation would be your own research guided by a professor in the field you want to study. However, you would get a degree in history but study and specialize in a specific time period/place. I would take a look at some postgrad university websites and talk to chairpersons of history departments for more info.

Different programs have different strengths and specialties in subfields, but all try to have a varied and diverse faculty covering many fields. Some schools have renowned 19th century America programs, others have great Latin American history reputations, public history, modern Europe, etc.Terminal masters programs in history aren't as important regarding your specific subfields interest since you wouldn't be authoring a dissertation, although you would certainly want at least one or two members of the faculty to have related research interest to your own. If you're looking into PhD paths, that should be wholly tailored to who specifically you'd want as an advisor and the archival resources that program possesses. It seems like you'd want a program with a strong 20th century US history reputation, so in that case you'd want to consider some of the best authors writing on topics you enjoy and find out where they work, make a list of different programs and different faculty members you'd like to work with for personal statements, try and find out if they're going to be taking graduate students the following year (literally email them, ask, and explain the prospective topic you'd like to write on), then polish your writing sample, get transcripts and recommendations in order and go for it. Don't worry that much about super specific research aims. A lot of historians stick with a narrow focus because it's what they're comfortable with and most fascinated in. There's a lot of people who diverge greatly on what they write on over time, and expand their research topics quite a lot. A guy at my undergrad entered focusing his research on the Protestant Reformation and ended doing modern European warfare.