My advice applies to the U.S.

What are the recent top books in your field? Who wrote them? Who are the leading scholars still publishing? Do some Googling; find out which prominent people teach at universities with history PhD programs. That's your basic list.

Don't just consider subfield and topic. Think about methodology, and opinions on controversial issues. (For example: if you're wanting to help 'write women into history,' do you believe this best happens by focusing exclusively on women, or by using women's and men's sources together to tell a broader story?)

Is it a "good program"? Look at the other faculty in the department. How high-powered are most of them within their field? Is the university one you've heard of, especially for reasons other than or in addition to football? History grad programs don't track with undergraduate "rankings" (which are manipulateable and biased towards schools with wealthy student bodies), but in general, you're not going to have an AWESOME PhD program at a uni with mediocre undergrad.

Probably around September-ish, contact the faculty you think might make good advisors. Ask them if they're accepting new advisees for the next year. Have a couple intelligent questions about the program, about the typical experience of one of their advisees (time to completion, and such).

And yes, in history, you apply to work with a specific professor but also to the program--since it's centralized funding rather than grants for specific projects (like in U.S. sciences or history in some European countries), a department admissions committee has ultimate say but obviously isn't accepting someone to work with an advisor who doesn't want them. So when you apply, you want your statement of purpose to stress why you're the right student for your advisor, but also why the program is right for you and why you're right for the program.