My fantasy ideas, when I first get them, are never very well-defined. I will have a scene or a character, but nothing else. There are obvious things about this character, but for every thing I am certain of, there will be a dozen I don't know yet.
It is, in many ways, like coming upon a big knot. You can see certain threads, and you know there's a splendid tapestry behind, but you first have to entangle them. You have to separate each thread from the others and see what it is made of before you can weave it back into a splendid work of art.
I don't think there's a single good way to do this. Each writer goes through his own process. This is only mine.
I start with the characters. It helps that most of the time, a character is the first thing I imagine, but even if I have a scene in mind, I start with the charaters. And I ask questions.
Who is this guy? What does he want? Why? Does he have family and friends? Where does he live? How does that shape his personality? Is there something peculiar about his appearance? If this is a scene, how and why did he get here?
Every answer to these questions should bring out more. Often, the setting will influence this character. No, wait, scratch that. Always, the setting will influence your character. If you know your world already, see where it connects with him. If you don't, see what could've shaped your character this way.
Always, ask questions. Pay close attention to the answers, especially the whys. Every time I do this, it's like pulling on a thread in the knot. Yes, some will tighten it and only make the big picture harder to see, but you will find the right thread, and the knot will fall apart.
There are knots along the way, though. Knots in the bigger knots. You'll find you have a beginning and an ending, and key scenes in-between... and yet, there is still that stretch in the story where you don't know what happens.
What do I do? I start with the characters.
How would he go from point A to point B? Isn't there someone that would be opposed to this? Who is that someone? What does he want, and why? Does he have family and friends? Where does he live? How does that shape his personality? Is there something peculiar about his appearance?
And thus a scene is born. I talked about ambitious characters before. This is why I love them.
Ask questions. Give answers. Explain the answers. Unravel the plot knot into threads, and then weave them back together to create your novel.