Use the voices in your head to dissect the idea, why this idea, why not others, how does it work, whats the value, benefit, experience, competition, barriers to make, barriers to sell, who is the customer, what makes it rock, who would buy it right now, why did they buy it, how do they use it, aspirationally connect it to other like ideas- its like x brand but with y, or what not. Just generate a crap load of notes about the idea.
Start grouping them on the wall and eventually into buckets and themes.
Lastly project yourself to the end game. Use a different color post its if you can. Think as if you've done the idea, its out, done, map that experience. Everything from how someone finds out about it, getting it, experiencing it, note the rewards of it, and does it end, how does it self sustain? Think ideal, but with a tint of reality based on what ya know.
Now step back, odds are you have a heck of alot more ideas up there than one you just started.
If you're hungry for more, get yet another color of post its and pick out the themes you see around the notes. There's probably 3-4 themes at work around your idea, bounce these themes off other people you know, if they resonate that alone could tell you you're in the running for making something that could work. Ideas always need constant validation, don't go at it alone, simply them and bounce them off people, friends, family, anyone. And dont get stuck on what they say, just record it, and save them for another wall session.