#1 - Aug. 13, 2014, 5:34 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 Heroic, and then use that option to enter. Kargath is in the arena at the start of the zone - if you get ported elsewhere, backtrack and you should be able to get ported into the arena as you approach the gate.
In your posts, please note your group size and makeup (high-end guild group, PUG, multi-guild team, etc.). 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.