#1 - Aug. 22, 2014, 7:13 p.m.
Once the zone is open, Nexus-Lord Donjon Rade (in Stormwind, Orgrimmar, Shattrath, Dalaran, Vale Shrines, or your garrison) will have a chat option to teleport you into Highmaul. Assemble your group, set the raid difficulty to Mythic, and then use that option to enter. You should zone in atop the hill and can proceed straight ahead to the Twin Ogron (the very first person to enter might be at the Coliseum and may need to zone out and back in).
Please refrain from using any silly scaled-up items (or at least let us know in your feedback if your raid was doing so) - we're in the process of fixing our scaling tech so that old items will be ignored, but in the meantime, let's try to keep feedback constructive. Uploading and linking to a log parse at Warcraft Logs, or any similar site that can handle 6.0 logs, would also be quite helpful.