Now that you have your brainstorming completed (although does brainstorming ever really stop?), it's time to complete your Synopsis! Now, if you're a pantser you may end up skipping this step, but planners like Kat and I like to have a solid plan before we go into a project. Here's how Synopses work and some tips on making yours!
What is a Synopsis? A synopsis is a summary of your plot that introduces the main characters and tells the ending. Synopses are used by agents and editors to gauge the strength of your story and make sure the plot will satisfy, but for our purposes we're going to use the Synopsis as a planning document. (If you're curious about official synopses, I actually did an article about this at Let The Words Flow a few years ago.)
How do they work? When you write a Synopsis you want to completely throw out the "show, don't tell" rule. All you want to do is tell! Share your main plot points, including not only action but the emotional states of your characters. Introduce only primary characters and talk about only the main plot path. Synopses are written in third person, present tense.
Wikipedia usually has plot summaries for major books and films. This one for Twilight is a great example of a simplified Synopsis, though if you were Stephenie Meyers you'd probably want to expand it even further.
Susan Dennard provides an amazing example of a short synopsis, what points to hit, and summarizes Star Wars Episode IV for us.
Here's another article on how to write a Synopsis in 5 easy steps.
If you want to, in the comments share the first couple paragraphs of your Synopsis. Bonus points: share up to the part where the main challenge is set and leaves the audience begging to know what happens next!