Halo 3

ok This is easy. Theres 6 skulls in the mythic maps 3 in halo3 and 3 in halo3:ODST Mythic heres a video by The smiley: Road2Recon sponsored by machinima.

For those of us who aren't fully versed on all the Halo lore, etc... I'll say this plainly so that you don't have to go hunting for the answer like I did.

In order to get all 6 of the Vidmaster Challenge: Brainpan skulls, *all you need* is Halo: ODST. I'm saying this so that you know you don't have to buy the Mythic pack for Halo 3. ODST has those maps on the second disc, and those maps do contain the 3 skulls from Halo 3.

So in other words, if you have ODST, you can get all 6 skulls there (very, very easily, and get yourself a quick 175 GS in under an hour), or you can use Halo with the two Mythic packs. All of the skulls are available in both places.

I highly recommend the videos that ryankelty posted above. They are excellent (and I don't want to repost what he put up :-).

Solution edit: As of 13th January 2022, the majority of online services for this game have now been closed so searching for the skull maps via my file share will no longer work. I have uploaded an alternate guide for the Sandbox Skull below.

For sandbox, you will have to overload the map with trip mines in order to access areas out of bound to locate the skull. On top of red base, spawn a trip mine and change the summary settings to 10 second respawn and run time minimum 1. Stand on top of the trip mine spawn, then keep releasing them and make a large pile without moving your player. Once successful the map will start to glitch and delete objects indicating it is now safe to access the skull.


The remaining skulls can be found using the below video guide.

You can only get three of these skulls at the moment.
Assembly
Orbital
Sandbox

You must be in Forge to get the skull achievements.
This goes towards getting recon in ODST if you get the other Vidmaster challenges.
The other three will come out with Halo:ODST

Look on youtube for the locations of the skulls and to make the sandbox skull easier go to bungie.net and search the file area for sandbox skull easy, there will be a teleporter to the skull and all you have to do is pick it up.