Uhm, I'd say... look around the web to find out what is the standard (or as standar as possible) format of a concept design document, if you haven't already.
Put down your ideas in that format.
Try to be as concise as possible, but also go as much in detail as you can: this is tricky, I know!
Look for companies you thing that might be interested and try to contact them by email.
Start small: it's highly unlikely that a Company like EA (for example) will consider your email.
Be patient.
Try to get an NDA or some kind of ageeement protecting your idea.
~ Mel ~