I assume you're in history? In that case, depending on your specialty, you don't, not in the way you're probably thinking. But that's okay!

I just graduated with my PhD in a very literature- and history-heavy field, in which pretty much every topic that can be easily gotten at as been gone over to death. The body of literature I work with is pretty much the definition of a closed canon. The historical events are well known. People have been studying this stuff for literal centuries. So even if your field is more open, hopefully my experience can give you a sort of worst-case scenario.

So, let's say you're in medieval history and you study the Black Plague. You might be able to find some kind of archival record that no one's published before, and do that, but more likely, you'll need to look at material that's already been gone over, maybe dozens of times, and ask yourself what all that previous work has missed. We know that Europe was far more multicultural in the Middle Ages than is usually depicted--do we know to what extent the Moorish or Jewish experience of the Plague was different from the indigenous European experience? What happens if we look at the Plague in the context of work on extra-regional awareness in the period? If you take a step in from the "1/3 of Europe" narrative, what does the Plague look like to a single family in a mid-sized city?

Look at theory, and not just within your field. Read the Plague through the lens of infrapolitics or political realism, or cognitive anthropology. Get postmodern: "the Black Plague is a modern catch-all term for what was actually a series of smaller and locally experienced occurrences of epidemic." How does changing that one way of talking about it also change the way you interpret more specific pieces of evidence?

When you're reading other people's scholarship on your specialty, what common assumptions, vocabulary, interpretations, etc., make you go, "Are you kidding? Obviously that's not how it works!"? Start there. Then think about the ways that your assumptions and categories might also be limiting, and read stuff that helps you think about your specialty in different ways. If you're interested in the history of Methodism in England, pretend "religion" doesn't exist for a second and see where that gets you. If you study the Vietnam War, try thinking about it as if the two "sides" didn't exist and see what kinds of power dynamics appear instead. Has anybody written on the effects the Magna Carta had on literacy and/or unmarried women?

(Probably a lot of these examples have been done, but you see what I'm getting at).

TL;DR: Unless you're in a specialty that has a large body of non-published primary material or the like, there aren't that many new topics. So get weird with it and push the scholarship on a known topic further, or in a different direction.