There is no order to most of the last half of the Main Quest. I've indexed it by trial, but that's just for my own convenience. The only "requirement" (see the Hiccup page for why it's not) is that you get yourself named "Nerevarine" by each of the four Ashlander tribes and get named "Hortator" by each of the three Great Houses. The order in which you do it (or if you do it at all) is entirely up to you.
You should have the Erabenimsun camp marked on your map. You'll notice that it's pretty much in the middle of nowhere, which makes it a real pain to get to. That's a foreshadowing of what you're going to have to deal with in the camp itself — a real pain.
The only civil person in the camp is the Wise Woman, Manirai. Everyone else will probably detest you only a little. At least until you start talking to them. Then they may or may not detest you a lot, depending on your Personality and Speechcraft skills. But that's OK. The only friends you need to win in this place are the wise woman and Han-Ammu, one of the gulkhans. Everyone else is expendable (as long as they attack you first).
How you proceed is going to be pretty much up to you, but you have to eliminate the entire current leadership: Ranabi, Ashu-Ahhe, Ulath-Pal and Ahaz. If you are 10th level or less or have a Strength of 60 or less, Manirai will subtly hint that you may not be up to the task of taking on all four. If you are 15th level or less she will subtly hint that you may not be up to the task of taking out Ulath-Pal and his bodyguard, Ahaz. She will not prevent you from attempting it, though.
All of these fights are going to take place inside of tents, so ranged and area effect spells are going to be out of the question. Plan these fights carefully. None of these guys should attack you automatically, so you'll probably need to taunt them into a fight.
At 94 HP, Ranabi is probably the weakest of the four, so you might want to take him out first. If you give him time, he will summon a skeleton, which you can fight or simply back out of the tent and let the spell wear off. He's got some decent magical stuff on him:
Ashu-Ahhe is the other gulkhan that is alone in his tent, so you'll probably want to take him out next. Aside from his Bonemold Cuirass, Mountain Spirit (Constant Effect: Shield 10 points), this should be a straight fight. Ashu-Ahhe has a Chitin War Axe (56% skill), 153 HP and is wearing Bonemold Armor.
Ulath Pal is going to be slightly different. First of all, he has a bodyguard, so this is going to be a two-on-one fight. Secondly, if you have killed either or both gulkhans, he should attack you as soon as you talk to him.
Once that little chore is out of the way, go talk to Han-Ammu, the only remaining gulkhan, about becoming Ashkhan. Turns out that Han-Ammu doesn't want to be Ashkhan, so you'll have to find a means of persuading him. Back to Manirai, who tells you that it is more a lack of self-confidence than anything else and you should persuade him to accept his responsibilities. Back to Han-Ammu and make sure that you have the Robe of Erur Dan, the Heart of Fire Amulet and the War Axe of Airan Ammu in your inventory when you talk to him.
If your Speechcraft skill is 70 or better, all you will need to do is show him the items and make a little speech with each one to bolster his confidence. He will then agree to become Ashkhan and will name you Nerevarine.
If your Speechcraft skill is not that high, you will need to give him the three items. On the bright side, he will let you keep the last item that you try to give him, so if there is one of those goodies that you particularly like, save it for last. He will then agree to become Ashkhan and will name you Nerevarine.
Now go back to to Manirai to collect the Seizing of the Erabenimsun (a Belt that casts a 50-point Telekinesis spell for 30 second) and you're done.