It took me a while, but it works like this: You make forts. Once a Herredcarl (the weird Fort Commander replacement without Fort Commander upgrades) reaches level 7, you can upgrade it for 100 gold into a city, transforming the Herredcarl into a Hurler (higher defense, inferior ranged attack) in the process.
Your cities are expensive but maintenance is much more manageable then, say, a straight-up gold penalty. New cities build Courthouses, Markets, Gambling Houses, and a Palace as your situation allows.
Aside from the 100-gold cost and the high maintenance the Jotnar are more or less as gold dependent as the Khazad, so prepare to work hard at economy. One thing to keep in mind is that most of the Kin promotions, despite their attractiveness, are things you really need to make sure you can afford before you pick them up.
- Stormkin is the one that (appropriately located at Trade) really pays for itself: it gives your 'Carls more range to farm barbs from, ensuring faster XP trickle, and it allows the Windtalker spell for your new city defender, which gives back a little of your gold. It also goes great with your siege, especially if you make them flying.
- Woodkin devastates the AI because they don't understand what to do about it, and serves as a superior Woodsman II (because your trolls already outspeed Woodsman II Recon Units...in ALL terrain...and can cross mountains) but it's not really necessary, your trolls are already terrifying.
- Seakin lets you win at the ocean, but the OO has had that nailed down forever anyway, and Krakens with Tsunami backup are still no joke.
- Firekin is meh - orcs tank fire damage, the Scorch is too slow to beat just building Sun nodes if you need it (and way too slow for offensive use), and deserts aren't where you want your trolls hanging out for extended periods of time anyway. Mirage is interesting, but a smart human player has scouts for your Woodkin-trolls already and the AI shouldn't be touching your workers anyway.
- Stonekin sounds like The Worker Promotion - workrate and defensive bonus! But you've got hordes of thrall militia helping workrate already, and your Citizens/Huscarls are tough enough to tank barbs without the promotion and not tough enough to deal with civ attack even with it.
Hunting should be your first tech without exception, preferably before your second troll is built. Spam trolls early, explore with extreme aggression, feed them on skeletons and orcs/goblins/frostlings while saving at least some of the wildlife for later capture, especially bears since they don't need a Carnival.
Once you've picked out three to five ideal city sites you need to fort them - NOW. The painful way is escorted Citizen teams who are going to be working without Construction for quite some time even if you rush it (which you need to, if you do it this way). The infinitely superior way is to start a war with your nearest neighbor, slaughtering his feeble early military with your absurdly powerful trolls (if you rush tracking this is a gamebreaker) but keeping him alive so you can farm Great Commanders and Jotnar-upgradeable slaves (the fastest way by far to get Citizens - does not work on the Clan, though, they just yield Orcish and Goblinoid Thralls). Great Commanders should then start forting all the ideal locations immediately, and by turn 150 you can have 5+ perfect cities if you're quick and good with economics.
Every path yields extremely good results for you. You can just stick with trolls if you like, they're very strong, incredibly mobile, get bonuses against everything found outside of a culture boundary...your melee line is berserk, your arcane line can build the Tower of Elements off the extremely beneficial Wonders you want to construct anyway and thus has easy access to whatever elements you prefer, your divine line is AMAZING despite not getting priests or religious heroes...
Your religion choice is important. There is a lot of pressure on you to pick ONE OF RoK, Order, or AV for their assorted benefits - you need a lot of gold income and Dwarven Mines, a way to deal with your massive maintenance costs directly, or at least massive city growth to compensate. A neat trick is to found RoK for the Holy City, build yourself as many Dwarven Mines as you can while you tech, then go with either Order or AV (I like Order, as I like being Good and Hell Terrain is really rough on your ideal cities) and just spread RoK everywhere you can. Trade the tech cheap or give it away, even.