Halo 4 World Championship Bracket System?

Im hosting a Halo 3 Singles LAN Tournament (double elimination) at the end of June and I would like to use a bracket system similar to the one used in Halo 4 World Championship. I’m expecting over a dozen players and many have requested a FFA element. I will have 8 Xboxs with flat screens so I was thinking of doing 4 person ffa the 1st & 2nd round and then singles for semi-finals and finals. Does anyone know how a bracket system like this would work or look like on paper? I’ve hired my friend to run the brackets so I will need to be able to explain this to him.