How do you get useful feedback on your writing?
By asking a specific question, because the default response to "what do you think?" is politeness, and politeness is useless.
What to ask instead: where did you get bored; where did you have to reread; what did you think would happen next; who did you think the story was about; what did you expect this to be about from the opening; and what is the one thing you would change. Diagnostic questions produce diagnostic answers.
The principle that makes feedback usable. Readers are reliable about their experience and unreliable about the cause. "I lost interest at chapter three" is data. "You should add a subplot" is a guess. Take the symptom seriously and treat the prescription as one hypothesis among several — the fix is frequently in a different place from where the problem was felt.
Who to ask:
Readers of the thing you wrote. Someone who does not read the genre will tell you the conventions are wrong when they are the point.
More than one person, since a single reader's reaction is one data point. Agreement between several readers is the signal worth acting on; a lone objection usually is not.
Not your family, unless they have proven they will be honest.
A writing group, which is the traditional structure and works when it has rules — reading in advance, the writer silent during the discussion, and a focus on what the piece is trying to do rather than what the reader would have written.
Beta readers for a complete draft; professional editorial assessment when you want a diagnosis rather than an opinion, which is a paid service and worth the money at the right stage.
How to receive it. Say thank you and nothing else at the time. Explaining what you meant is the most common mistake — if it needed explaining, that is the finding. Sit with it for a week before deciding what to act on, and expect to reject some of it deliberately.
When not to seek it. Too early, when the work is fragile and undirected, feedback can kill a draft that simply needed finishing.