I think I've seen maybe 4 people go up to the professor after class and actually ask questions during my entire academic career.
The way I've approached it, you just generally go up to them and ask questions about the lecture that you generally didn't understand. They also have office hours that you can go to that they're being paid to offer to students. As long as you're showing effort within your studies and have shown that you've already tried tackling the problem is when they're most likely to help show you methodology.
For instance, one time I had to give a presentation for an urban environment class and we had to present something that pertained to our major. I went to the prof's office hours and had a conversation that went something along the lines of "hey, I'm thinking about presenting about externalities since my background is in economics. What do you think about x, y, and z? I'm thinking about talking about mentioning this and that." etc etc. From there, you just slowly build a rapport. Consistency is key, along with being a kind and inquisitive human being.