You just have to beat the last mission on legendary with the iron skull on and with 3 other people and insted of using the warthog u use the ghosts. The ghosts are under the bridge yo walk up to get to the battle before you battle 343 guilty spark.(You must beat 343 guilty spark before getting in the ghosts.)
right now there are 4 vidmaster achievements but when halo 3 ODST comes out there will be a total of 7, which means 3 more will be coming out. check bungie.net dont know where it said i but im sure you can find it.
Make the most out of the save points, kill every enemy as you progresss and checkpoints will activate. If you blaze through the level and fail to kill every entire wave of flood you will skip the checkpoints.
As reference I activated 7 checkpoints before getting to the second floor in the initial fight with the flood.
dont do it with someone who when shot at hides in the corner shouting STOP SHOOTING MEEEEEE!!!! and finds every hole possible when in ghosts . . .. also this took me 2 friends and a retard 4 - 5 solid hours of playing to complete . . . goodluck