Onyxia Attunement

Horde

• Go to Kargath in Badlands in Eastern Kingdom and speak to Warlord Goretooth in the tower. Keep asking him questions until you get an item. Right click the item to start the quest "Warlord’s Command".
• Go to LBRS and kill Overlord Wyrmthalak, Highlord Omokk, and War Master Voone. Also retrieve the "Important Blackrock Documents" that randomly spawn on the ground beside either Wyrmthalak, Omokk or Voone. If you don’t know how to find these bosses, get a party that knows. You can also use the addon "Atlas" to get maps of all instances.
• Return to Warlord Goretooth in Kargath to complete the quest.
• Get the next quest from Warlord Goretooth called "Eitrigg’s Wisdom". You are to find Eitrigg in Orgrimmar. The coords are 34;39. If you don’t have an addon for coords, go to Grommash Hold, the big building in The Valley of Wisdom, and find Eitrigg inside. He will not be marked on the mini map, so just look around at the NPCs. Speak to him and he will send you to Thrall, who is just a few yards away in the same building.
• Get the next quest from Thrall called "For The Horde!". Go to UBRS and kill Warchief Rend Blackhand. You will need a raid of 10 for this. Get his head and return it to Thrall for a nice trinket.
• Get the quest "What The Wind Carries" from Thrall. All you have to do is ask him some questions. Complete the quest.
• Get the next quest called "The Champion Of The Horde" from Thrall. You have to speak to Rexxar who wanders around the Desolace map (on the roads) and down into Feralas abit too. It may take a lot of running around before you find him - but he is there!
• He gives you the next part called "The Testament of Rexxar". Go to Sorrow Hill in Western Plaguelands and find Myranda The Hag. If you got coords, she’s at about 50;77.
• She gives you the quest "Occulus Illusions". Go to UBRS and kill dragons and collect their eyes. Find a raid and you’ll collect them quickly. Return to Myranda the Hag when you have 20 eyes.
• The next part called "Emberstrife" is easily soloable even though it’s an elite quest. Just go to Dustwallow Marsh and find the southernmost cave located at 54;84. Wear (and use) the amulet Myranda gave to you to turn into a dragonling and be able to talk to Emberstrife. He will give you three quests, so it’s a good idea to have enough free slots in your quest log. You have to kill three dragons and you can do them in any order.
• "The Test of Skulls, Scryer" Scryer is in a cave in Winterspring. Coords for the entry are: 57;49. A party of 3-5 will easily take him down. Be sure to loot his skull.
• "The Test of Skulls, Somnus". Somnus is in Swamp of Sorrows. He is on patrol, so he moves around (note: on this private server he likely doesn't... we found him in the south-eastern part of the zone - Nomagio). 3-5 level 60s will take him down quite easily.
• "The Test of Skulls, Chronalis". Find the dragon near 64;50 in Tanaris, at the Caverns of Time.
• Return the quests to Emberstrife and get the next quest in the chain: "The Test of Skulls, Axtroz".
• Travel to Wetlands and find and kill Axtroz near 81;47. He is slightly harder than the others, but 4-5 people can kill him without breaking a sweat. Return to Emberstrife with Axtroz’s skull.
• In the next part, called "Ascension…", you seek the advice of Rexxar again. So go back to Desolace and travel the road between Stonetalon and Feralas to find him. He will send you on a quest called "Blood of The Black Dragon Champion".
• Go back to UBRS and kill Drakkisath. Be sure to loot his blood off of his corpse. Then return to Rexxar and turn in the blood. He will give you the Drakefire Amulet that you need to get into Onyxia’s Lair. Congratulations, you’re done!

As for Onyxia’s Lair, it’s located in Dustwallow Marsh at 53.77. You need to wear the Drakefire Amulet or have it in your backpack, be in a raid and be at least level 50 to enter.