I like to write a quick summary of the idea. It could be a couple of paragraphs about the story, but usually it's more like a page or two. There are a lot of variables that can determine how long the summary turns out to be. If the story is a stand alone it might just be a few paragraphs. If the story for part of a series then it'll be much longer. If it's part of an on-going series I need to figure out where the book fits.
If this is the first book in a new series then I'll need to work up the series arc too. This can take a bit of time, but it's worth it to write it all out for me to see.
I'll use the summary later to write the synopsis, but for now it's just for me to get my thoughts in order. And to get ready to move onto the next step. What's the next step?