User Stories, though simple in concept, are hard to do. The reason is simple - we are not used to thinking in terms of “delivering thin slices of end user value” and so it means we have to retrain our mind from the way we thought about the problem to the approach we want to take now (see The Backwards Bicycle to understand why this is so hard.)
What this means is that the best approach to learn how to do User Stories is the practice writing them. And it helps if you practice under the watchful eye of someone who can help steer the conversation, someone who has done it before. If you don't have that person, start asking the questions you see above, starting with “When we have finished this work what will we demonstrate to our end user / customer and what kind of feedback do we expect to get?” If you cannot answer this question, or the answer is “we will show the schema” then in most cases you have a big task, not a user story.