I did one in the humanities, full time and funded. A while back.
Do you have a clear idea of your chosen topic? I still remember well my intake interview where they basically wanted to know what my final argument would be, which I thought was a bit unfair as I hadn’t done the research yet! But I am aware that even in humanities things have changed a bit and it might be possible to get accepted onto a predetermined research programme. But still, what is it you want to argue?
Secondly, are you absolutely passionate about your topic? As others have said, a humanities phd is a long, lonely slog and if you’re not obsessed with your topic you’ll probably drop out.
Thirdly, getting the right supervisor is absolutely key.
Fourth, In your position I might try and do a Master’s first, either taught or possibly an MRes or something with a bit of research training and experience in it. And if you don’t, ask about what support and training you will get in research methodology etc at the start of your research: when I did my doctorate we were basically thrown in to writing 80K words with no support on how you might do this (beyond the individual supervisions, of course). But I think that’s thankfully rare these days.
Finally, I agree with those who say don’t bank on getting a career out of it. It’s very very difficult to get an academic job these days and most of those I know who did succeed in getting one (I didn’t) are unhappy. I think there was some research at one point which showed that humanities phds actually lowered earning power across a work life because it took years out of the careers of bright people, usually at the beginning, so they were behind their contemporaries on the career ladder. As I said, I didn’t break in to academia as I had at one point hoped but I have had two careers since which I have very much enjoyed and continue to do so, and I draw on the skills and tenacity I learned from my phd all the time, and it was a topic I loved and I don’t regret it at all. But there’s no way I could do it all again now I’m a bit older, either financially or in terms of the concentration required. But I know others do, so good luck if it’s what you really want.