expressions - Is "How do you ... ?" a polite question to ask the right way to do things?

Responding to the broader question you put, it wouldn't be polite to ask "How do you spell xxx?" before a polite introduction. Generally, this would include a greeting and some small talk. The switch to the question can be smoothed by a hedge such as "Can I ask you a question - ...".

Some might say that "May I ask you, ..." is preferable here, but that would sound stuffy in casual conversation. And while "How does one spell xxx ?" is probably logically preferable for the reason you suggest, it sounds ridiculously formal. "How do you spell xxx ?" is preferable by far - the 'you' is not stressed, in fact, hardly pronounced ("How d'yə spell xxx?" - cf "How d'yə do?") - to show that the 'you' in this case means 'people in general'.