I suppose the answer to this question depends on how you define "historian", but I'm going to assume you mean a professional historian in the capacity of either a college/university professor or a museum historian/librarian/archivist. All of these jobs would require you to develop advanced historical skills, meaning conducting research with primary and secondary sources, interpreting those sources, forming hypotheses and conclusions based on your readings of those sources, and then having the writing and communication skills to communicate those conclusions to your audience. In most cases, you'll have to develop foreign language skills to enable you to read the sources you need for your research, as well as other research skills that may be relevant (e.g., statistical modeling). If you were to become a professor, you would also have to develop your skills as a teacher; if you were to work on the museum/library/archives side of things, you would have to develop the specialized skills needed for those jobs, such as curation and preservation of historical artifacts, or cataloguing of books and documents.

All of these jobs require both an undergraduate and a graduate degree in history. Becoming a college/university professor generally requires a Ph.D., while jobs in the museum/library/archives fields generally require an M.A. (or a specialized degree like an MLIS degree, in the case of librarians), as well as specialized training for whatever role you would be performing in that job (curation, preservation, etc.). Either way, you're looking at between six and ten years spent in school obtaining the necessary credentials to get these jobs.

As a college/university professor, you would probably find yourself teaching between six and ten courses a year, depending on the type of institution you're working at. If you're at a research university, you would also be expected to continue carrying out historical research and publishing books and articles based on that research; your ability to obtain tenure is dependent on those publications, as well as your teaching performance. Outside of the university setting, research and publication may or may not be part of your job, depending on what career path you take, but obviously would have other duties relating to your main job.

And now for the part of the answer that I hate to give, but find myself giving a lot lately. The problem with following this career path is that these jobs just don't exist anymore. The job market has shrunk dramatically in the last decade or so and hasn't shown any real sign of recovery. In the old days, the pathway was simple: you got your Ph.D., you got a tenure-track job, you published your dissertation and another book, you got tenure, and you were set for life. However, the tenure-track jobs have all but dried up as history enrollments have dwindled due to our university system prioritizing STEM and business-oriented fields over the humanities. Now, when a tenured professor retires, instead of hiring a new tenure-track professor to replace them, universities are hiring adjunct professors (part-time instructors who generally get paid by the course and have few or no benefits and no job security) or non-tenure-track instructors, or not hiring anyone and just foisting those courses on the existing faculty. The few tenure-track jobs that do open up nowadays generally get hundreds of applications because there are about three new Ph.D.s produced for every new job that opens up, and that doesn't even count the existing Ph.D.s from previous years still job hunting. If you try to go into college/university teaching at this point, you are almost certain to get stuck on the adjunct grind, barely eking out a subsistence-level existence rather than an actual career. The prospects aren't much better on the museum/library/archives side of things either, as those jobs have also taken a hit in recent years. The history job market tanked after the 2008 recession and never really recovered, and has taken yet another hit in the last couple of years due to the pandemic.

To put it bluntly, it is very unlikely that you can start on a career path toward becoming a professional historian and succeed in making a career out of it in 2021. I'm sorry if this isn't the answer you wanted to hear, but it's better that you know what you'd be getting yourself into up front than spending the better part of a decade in school and then finding yourself without a job. Studying history is rewarding and I love what I do, but I can't recommend that other people follow that career path in good conscience, knowing what I do about the job market from first hand experience. It sucks, and I hate it, but it's the way it is.