If I want to ask someone how they learned English, is the sentence below correct, or is there a better sentence for it?
I probably wouldn't use that. I might say:In which way you have learned English?
Without more context, I'd understand that to mean "What is a good way to learn English?" That is, I'd hear you as the unspecific pronoun. If the listener is still learning, I'd ask How are you learning English? or, more likely, How are you studying English?I would say 'How do you learn English?' if the listener is still learning it.
The second sentence is a non sequitur. It sounds like you're posing a general question, What's a good way to learn English? If you want to know what technique your interlocutor uses, put the question in the present continuous.I think the context could be 'You speak fluent/broken English. How do you learn English?'.