#1 - Sept. 4, 2014, 7:03 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.
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.