So, here you are at step 1, which is to summarize your story idea in one sentence. Wow, talk about hard! How can you distill down all of the action involved with chasing that terrorist, or the emotional connection your lovers share, or the sweat-inducing pressure your bomb diffuser professional is feeling? Well, you're going to have to do it, because that will be your starting point. This is where you will start to grow your story, from this little seed of a sentence.
I can already hear some howls from the group (wait, is there a full moon?). But I have all of these great ideas, and now I'm just supposed to forget them? Heck no! Keep a list of whatever ideas you have for your novel, because you'll be able to incorporate them later on. And as you come up with more things, such as great lines, scene details, etc., keep those written down as well. They will only serve to help you out as you go along in the process.
Okay, there are a few restrictions with this first step as well. And you may need to work through multiple iterations of the sentence to get things whittled down to a concise, short sentence. This sentence should ideally be shorter rather than longer, 15 words or less if at all possible. Also, no character names at this point. Sure, you've got what you consider is a main character at this point, but what about later, when you realize that the antagonist that you thought was more of a minor character suddenly becomes a focal point in the story line? This step should take around of hour of crafting, but could take longer or shorter.
I think that I worked on my single sentence for at least a solid 30 minutes, probably longer. It worked at the time, but as I'm going further in the snowflake method, I believe that I'll have to revise this to reflect the changes that have happened to my story along the way. This is a good thing, and means that your story is getting deeper, better.
So what are you waiting for? Start working on your single sentence for your story. Take time to enjoy the process, and remember to write down the other ideas that come up while you are crafting this short story synopsis. Step 2 of the snowflake method will be building on what you do here.
Oh, and I'm sure you're curious what MY step one sentence is. Well, here it is:
Two best friends, geocaching in the Wisconsin woods, become entangled in a voyeur's murderous game.