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.
The Zainab camp is southwest of Vos and should already be marked on your map. Unlike the Ahemmusa, the Zainab have an Ashkhan, so feel free to just drop in and have a chat with Ashkhan Kaushad. If you have completed the side quest about opening trade with the Zainab (from the guard captain in Tel Vos), you already have an idea of how difficult it is to deal with any male in the camp. The women are fairly easy to deal with, so hold them in reserve if you need additional information or help.
Kaushad will not even want to talk to you unless he likes you (disposition of at least 70). If you search out the Zainab Wise Woman, she'll tell you that he's got a thing for Outlander Shoes (Exquisite, no less), but plain old bribery will work, too. But just because the old boy likes you doesn't mean your job is even half done. Oh no.
Kaushad is not sure that you are worthy to be named Nerevarine, so you'll need to ask him to set you a task to prove your worthiness. How about killing a vampire In the Nerano Ancestral Tomb?
Finding the tomb is relatively easy. Head west to the mountains and then go north, past the Nchuleft ruins and look for a tomb entrance on your right. It will have trees on either side of it and faces west. Expect the usual tomb creatures in addition to the vampire (pack along some Restore potions/spells/items) and then head back to camp. Ready to be named Nerevarine? Not hardly.
Kaushad now wants a gift. But not just any trinket or bauble will do. He wants a bride. Not just any bride, though. He wants a high-born Telvanni bride; pretty and plump with big hips, no less. Now which high Telvanni do we know that might have a daughter to spare? I'll save you the trouble. Divayth Fyr doesn't even offer you the dialogue option.
It's time to go talk to the women. The Wise Woman will tell you that no sane Telvanni would even consider it. But she says there is no reason why you couldn't make Kaushad THINK that he's getting a high-born Telvanni and sends you off to see Savile Imayn, the slavemistress in Tel Aruhn.
Before you do that, though, you've got a couple of details to iron out. First off, you're going to have to escort her back to the Zainab camp. That means taking care of any pesky critters (like Ashlander Outcasts) along your route. If your level is high enough, that might also include a few respawning daedra (I've run into Golden Saints, Winged Twilights and Scamps) as well as the usual wilderness critters. So plan out your route carefully and, whatever you do, DO NOT DISPOSE OF THE CORPSES. Speed is not nearly as important as getting your cargo to its destination in one piece. A Heal Companion spell can be a tremendous help.
Secondly, your bride must LOOK like a high-born Telvanni lady. So head for the clothier in Tel Aruhn and buy Exquisite Shirt, Exquisite Skirt and Exquisite Shoes. Also, your bride must SMELL like a high-born Telvanni lady, so stop by the apothecary (in either Tel Aruhn or Tel Mora -- they both have it) and buy some Telvanni Bug Musk.
Now go to the slave market. It just so happens that Savile Imayn has just the right girl for you (what a coincidence). It also just so happens that she won't sell the girl to you until you bring the shirt, skirt, shoes and perfume. But you've already taken care of that, haven't you. Now you can get down the the haggling. Imayn wants 1200 gold for the girl. If she likes you, you can get her for 1100. If your Mercantile skill is higher than 40, you can get her for 600. Once you've closed the deal, go open the girl's cage (her name is Falula Llervu) and give her the stuff. She will now follow you back to Kaushad's place.
Falula (what a name) is a decent enough swimmer, but hasn't got the combat skills to be able to effectively swat a fly. You'll need to clear out the Slaughterfish and Dreugh between you and the coastline or you'll lose your bride-to-be pretty quickly. And if Falula dies, you'll get one of those dreaded "this path to your destiny is closed" messages. This might fall under the category of a "bad thing," so protect her well.
Kaushad is pretty happy (though he does mention that you should have found one with bigger hips), names you Nerevarine of the Zinab and give you a hunk of butt floss (a thong) to seal the deal. Mission accomplished.