I am more of a dilettante than a Renaissance man, but I am interested in almost everything and read widely in history, science, literature, etc.
However, I can't imagine anything harder or less painful than trying to become a Renaissance Man (or even a dilettante) because that's your goal. Is suspect most learned people got that way because they just studied lots of things that interested them -- not because they wanted to be learned.
I love learning. I love the feeling of learning. I love the "journey" of learning. If I'm not actively learning something, I'm bored. When I'm looking for something to do on a Sunday afternoon, I pick up a book or listen to a podcast about something I don't know.
I do not think I'm much smarter than average, but I do notice one big difference between me and many of my friends: learning has no baggage for me.
* Learning is not competitive for me. I do not feel the need to learn more than anyone else or faster than anyone else. While, in theory, a competitive nature might spur one to learn, what I see most often is that competitive learners give up when they fail. If they don't learn, say, Calculus as fast as the rest of the class, they figure they have "lost" and never try again. Don't learn something to prove that you're smart or to show off that you're smart; don't learn to "impress the girls." Learn to learn.
* Learning for me is morally neutral. I don't feel that there's anything one should or should not learn. I have read a lot of classic literature. On the other hand, if I don't feel like reading "Moby Dick" or "King Lear," I won't read them because I feel like "one should."
* I don't care about being smart or original. I don't learn a subject in order "to be smart." I learn a subject because I'm interested in the subject. If I stop being interested in it, I stop studying it.
I'm a writer and theatre director. When I create, I don't care at all about being original. I care about the subject or story I'm trying to tell.
* I am not phased by brain fog. You know that feeling that comes over you when you're studying a hard subject? It's a sort of exhaustion. The subject gets farther and farther away and you have to expend more and more energy to keep up with it. Finally, you shut down. Most people I know are really daunted by it. I'm not.
I experience it as much as anyone. And I dislike it. But I've learned not to try to push past it. Since learning for me isn't competitive or a moral urge, when I get brain fog, I just stop. Later, when I'm more alert, I return to the subject. Sometimes "later" is weeks or even years later. That's fine.
So the question is, how does one get to be the way I am? How did I get this way? I don't have all the answers, but I know one of them: by ignoring school.
Unless you went to a really good (and really atypical) school, your school approached learning in all the wrong ways: it made learning competitive and goal driven (e.g grades); it made learning a moral imperative ("You SHOULD read Shakespeare!"); it made learning stick to a time table ("You must 'learn' Algebra by the end of this semester or you fail!").
Wrong! Wrong! Wrong! Beyond wrong. Deeply detrimental.
Since most "smart" people endured close to two decades of traditional schooling, it's no surprise that they are indoctrinated in those bad methods of learning -- methods that make learning not fun yet make people value learning, so that in the end you wind up with people who hate to learn but feel that they SHOULD learn.
You wind up with people who to to HAVE learned but who hate the process of learning itself. They want to tick off subjects they know on a ledger, and if they could press a button and just know all the subjects without learning them, they would, because school has taught they that the journey sucks.
Luckily for me, I always hated school and never took it seriously. I put up with it when I had too, and did all my real learning on my own.
How do you get to that stage? I don't know. If you're already in school, think about what messages it's sending you about learning. Is the process itself joyous? Or is it all about the carrot (the A) and the stick (the F)? If you're past school, think about what messages it sent you and how whether you can free yourself from them.
In the end, I believe that being happy is way more valuable than learning. Learning makes me deeply happy, which is why I do it. If it doesn't make you deeply happy -- if the PROCESS -- doesn't fill you with joy, don't do it. Life is too short!
If you love learning, you don't need to ask how to be a Renaissance Man. The answer: just do what you love.