In the second of our "mentor advice" threads, this year's mentors are offering their thoughts on how to flesh out concepts and identify goals. If you've got an idea and you're not sure how to break it down, you might find some helpful approaches here.

jotson

Make it work as quickly as possible. Don't worry about how it looks or how bad the code is. Just make it work and see if it's any fun. Then polish once you've decided it's good.

Akien

I'm not a very good game designer, so I don't have a lot of personal experience to share here. But the common advice is to keep your scope small, and try to narrow down on one, or max two concepts which you can develop sufficiently during a jam. Try to get your game loop done early on (and make sure it's fun!), and iterate on that.

And even for your one or two must-have main concepts - be prepared for having to change plans as you face technical issues or lack time.

Trevor

I like to pick "big idea" concepts that can be explained in no more than a sentence or two, but with interactions which are focused on a small number of verbs. 

Cheeseness

I like to start by paring back an idea to its essence - what is the bare minimum of content/gameplay/narrative that would convey the core experience.

Taking the time to spell that out in detail (explaining it to a friend almost always highlights something I've overlooked) helps me see what's essential. Once I've got a handle on that, I can focus on building out things that explicitly support, setup, enhance, are resonant with, or improve the pacing of that core experience.

StraToN

Make a prototype and apply KISS principle : Keep It Simple Stupid.