First, in that story you want to read, who is the story about? Don't try to understand who that character is, but consider that character's values, their strengths and weaknesses, and why you want to see that character on the page. What about that person is worth writing about? People have endless lists of qualities, but what should go into the story?
The next step is just like every other part of story creation: What is the conflict? What is the problem you want to see that character face? Don't just think about what problem they could face - that's easy. What makes you want to see your important character go through that particular problem?
At this point it should be getting difficult. If you really care about this character as the one you want to see in a story, putting them through a really difficult problem might feel a little personal. After all, this is the story you want to read, so it needs to be a real page-turner.
This is now where Toni's advice becomes difficult to put into play. As our character faces the problems and challenges, we now have to write a bunch of that character getting beaten up, knocked down, and forever facing the uphill climb. Why would anyone put themselves through the process of doing this to a character? For those who are writers or who follow this blog, the answer is simple: the outcome.
When you think of that story you want to read, what is the most amazing, satisfying wrap-up for the character's adventure? Don't even think about the problems you've already established. Just think about how this concludes, and why that particular ending brings you such joy. Knowing the how and why of the ending should be conclusive statements referring to the how and why of this character's beginning. At that point, you have established the tent poles of the story arc.
At this point, everything between the beginning and the end is your choice. You just need to know how they fit between the points you've established.
The only question left to ask now is why you aren't writing the book. You literally have everything you need.