Your answers are fine and perfectly natural. Please don't think there's a "secret answer" that only native speakers know. You're already doing it right! :)

There are 2 ways to answer your question. First, you want to know how native English speakers answer this question. Technically the common answer is something like "Good thanks". This is because it is not a real question, it is more of a greeting or a conversation starter. Even if you are bad, most of the time, people don't care. They are only saying it because it is a common thing to say. It is like when people say "Good morning" instead of "Hello", even if the morning is not good at all. Everyone asks this question, in formal and informal situations, between friends, cashiers, and at formal professional meetings. It is simply a polite greeting, and when it is asked as a polite greeting, people usually say "Good/fine thanks," whether or not they actually are fine. They might also add "And you?", and the first person will say "Good thanks." Then they will start to talk about what they actually are meeting to talk about. This is why I really hate this 'question'. The other way to answer your question is this: If you consider this to be a real question, and you want to answer honestly, AND, the other person actually cares, for example, a doctor, then you can say something different, like "Not good" or something. In ordinary settings no one would care if you are bad, and this kind of conversation ends quickly. For example: Cashier: Hi, how are you today? You: Not good. Cashier: Oh, no, why? You: I missed my train this morning, got to work late and left my key at home. Cashier: Oh I am sorry to hear that. Tomorrow will be better. "How are you?" is what we call "small talk" in English. It is a conversation starter, a rhetorical question where the asker isn't really interested in an answer other than "Fine". They are not interested in a detailed response if the answer is no. If you say no, they won't want to stay around listening to you all day. Like my example, they would only want a quick simple summary of why you are bad, for the sake of having a 'conversation'.

Well there's no set answer. It literally depends on how you're feeling or what you're doing at that moment. Q: How are you? A: Good thanks, you? A: Not bad. You? A: Pretty crappy if I'm honest. What about you?