Oh, why hello there! Welcome to my third guide, I decided to make this guide for 3 reasons.
1). I wanted to make a guide of my favorite tank.
2). I wanted to share my build.
3). I wanted to share my trolol forever slow build.
In this guide I'll basically.. Well, I don't know, just keep reading..

Pros
-Super slow when using the troll build
-Super slow even without using the troll build
-Has a great ultimate, basically stuns the whole team when used properly and if the enemy team is bunched
-Pretty beefy, like a tank, durr.
-Can 1v1 decently against someone that isn't lifestealing. l0l

Cons
-I really haven't thought about her cons.

Flat health marks ftw. Why? Because I said so. Why? Because health is awesome, and it's good for the jungle in the beginning.

Flat armor seals ftw. Why? Because they help your armor and they also help your jungling in the beginning.

MR/lvl glyphs ftw. Why? Because casters are pain in the *** late game, maybe?

Health quints ftw. Why? Because they give lots of HP and it's wonderful! Same reason for the marks.

I take 1/29/0 because I get improved Ghost in the offensive tree, while getting the main tanky masteries in the defensive tree. Make sure you always grab the health, the more health you have, the more damage your Northern Winds does.

Normal Tanking Sejuani
Look, you are supposed to modify this build as the game goes.. If you're playing pubs, and you have an enemy team full of AD, get only Armor and ignore the Magic Resist. Vice versa when playing against an enemy team full of AP, get only Magic Resist and ignore the Armor.
My build is more for a team that is balanced with AD and AP. I know what some of you people may say, "Oh this dewd's got HoG and Philo Stone like Phreak, he's n00bzorz!"
No, I'm not a damn n00bzor, you're a tard and don't understand the concept of it. The reason why you get the double Gold Per 5 seconds is so that while you're ganking and what not, you're making money still, and if you fail a gank, at least you won't be dying of gold, and if you succeed in a gank, you'll have tons of money. It's just a method of rushing to your bigger items.

Trolol ForeverSlow Sejuani Build
Ahh.. This build is amazing. I love it in every aspect. It's not too beefy in Magic Resist or Armor, but it's got a ton of HP. That Frozen Heart attack speed slow, Rylai's Crystal Scepter slow with your Arctic Assault and Permafrost stacking, along with Frozen Mallet and Frost stacking on the right click, there's so much slow you can make someone rage quit. Oh, and you won't need anyone else to CC once you get all 3 of these items. You're basically snaring them with how much slow you've got.

I shouldn't need to explain this. Permafrost and Northern Winds are maxed out ASAP because that's your 2 main spells, the Permafrost's slow increases the more you upgrade it, and Northern Winds does more damage based on how much HP you have. I keep Arctic Assault at level one because I don't want higher mana costs, and I don't think the lower cooldown is worth it.

Sejuani can lane if you want her to, but I don't like doing it. I like her more as a jungler, roaming and ganking all the time.
Start at blue buffs, hit the wolves, wraiths, red buff, clear the dubs (twin golems) and then head to top for a gank. If your teammate over pushed it, don't gank, head mid. If that's over pushed by your teammate, don't gank, head bot, and if -THAT'S- over pushed by your teammate, don't gank, and tell them to back off for a bit so you can gank some where. In the meantime, keep jungling, or help push a lane instead, if you want, which I don't really suggest.