Okay, so I've looked around at all the sejuani builds on here, i've played a lot of them...and they DO work. there's only one problem though: what do you do when your team doesn't know how to play? (if I offend anyone by this, my bad...i just got out of a game 12/5 and we lost because it was basically 2v5...) at any rate, i will tell you now, you will NOT want to use this build in a ranked game (as of yet I haven't had the chance to test it) but if you have to build tanky AND ap, I hope that this gives you everything you need to know. Also, The pages for item sequence builds and runes are bugged out, so i will have to just write those out in the build guide below. One last thing: if you a jungler, I want your opinions on some early game builds. late game, this should cover you, but I jungle really slow with sejuani because I don't exactly jungle a lot. So, give me a shoutout, and I will include a jungling section to this guide with credit due to you.

The masteires I have above are generally what runs best for sejuani. You want health, armor, and m.r. for survivability, but remember that since sejuani scales off of ap, you need some to start with too. it also helps to lower your cooldowns, because you will need to spam some spells here.

okay, sorry for this, but my page seems to be bugged. that being said, i'm going to have to write all of the runes down instead of just finding them on screen (this annoys me as much as it's GOING TO annoy you, trust me...)
Alright, so for marks, you want magic penetration runes. this gives your abilities some decent kick early game.
For seals, You want armor. it can be armor per level, or flat armor, but personally, i prefer the per level runes. You also want magic resist glyphs, and these can again be per level or flat m.r. The point of this is to give you as much survivability as possible throughout the game.
The quints can be changed a bit. It depends on your preference for fighting, and it depends on how good you are at your prefered style. Usually, Health quints are the best choice, but from trying other builds, AP quints also work. Again, this depends on how you want to play your game, and how well you do by playing in that way. if one way doesn't work, try the other.

This is the other bugged part. Your build as an ap off tank can vary...a LOT...i'm going to put down some BASIC items that i've found useful and a couple of ways to start.

Starting items: Regrowth pendant + potion (this will turn into a philosopher's stone really quickly), Doran's ring, Doran's shield.
the basics to this comes again down to your style, but for the most part, i find option one more useful. you have great hp regen early game, and once you upgrade, you get bonus gold over time. It makes things a lot simpler later when we go for bigger items.

Early game:
Boots: This depends on how you're doing. If they have a lot of ad, get ninja tabi, or merc treads for heavy ap. if you're doing really good at the start, consider buying sorcerers shoes or lucidity shoes to keep snowballing the other team.
Tear of the goddess: this does two things. One, it quickly gets rid of any mana problems you have. and 2, it upgrades into a valuable item later.
Heart of gold: this item gives you some bonus health and even more bonus gold over time. this is useful for quickly going through your build. If you're just murdering the other team completely and getting overly fed, go ahead and finish the tear into the next item before getting this.

Mid game:
Archangel's staff: Tear of the goddess just paid off. This item gives you all of its bonuses, AND bonus AP based off of your mana. now tell me how this could POSSIBLY be bad.
Randuin's Omen: now you have some more health, some more armor, and in case your NORMAL slows don't QUITE get rid of that annoying ashe, you now have a chance to slow them when they hit you as well. As an added bonus, this helps you FURTHER slow enemies in team fights by activating it.
You CAN upgrade the philosopher's stone now...but i find the gold bonus better to keep through your build.
Late game - some big and pricey decisions:
Thornmail/warmogs/force of nature: this depends a lot on who you're up against and how you're handling them. If you're against a lot of ad (yi, vayne, ashe and nasus in a single game for instance) definitely take thornmail. the armor bonus helps, and the damage reflect quickly depletes any real chance of them living if your team goes in to fight with you. however if you're fighting a lot of magic champs (annie, ryze, malz and maokai) take a force. the magic resist and health regen make you virtually invincible to them, and your team can easily kill them all after they run out of burst. If you're fighting a balanced team just take a warmogs. the MASSIVE hp bonus and health regen will keep you alive through almost anything.
Rabadon's vs. Guardian angel: this is it. the build is close to done. the game is almost over. but one item can make or break a game. If you're still somehow dying even though you've followed this build, or if you haven't quite COMPLETELY followed it but have a lot of resist and armor and hp, then get a guardian angel. this will let them focus you, die to your team, and then rez you so you can troll them for the few remaining minutes of the game, (why are they focusing the tanky one anyway???) if you're WRECKING the enemy team, get a trabadon's deathcap. this will make you the most powerful tank in existance, and the only thing that I can think of that would stop you is a team full of aoe stuns, snares, and...well...you get the point. which brings me to my last piece of building.
Reverie vs. Eleisa's miracle: If you're having trouble with cc, or if you just get annoyed when your teammate dies while you're slowed, stunned, snared, silenced, feared, taunted, and knocked up for the whole time that theyre being chased (and no, YOU can't die from all that...you're the tank...kinda...) get the miracle. tenacity is actually a VERY underrated way of getting out of cc and living with basically no hp. if you arent having that much trouble, the reverie is better, because the cd reduction and the movespeed active give you the ability to chase all day. if you find it better to up this instead of heart of gold eariler, that's fine...but keeping AT LEAST one of the gold items until the end makes sure you can build quickly.

The two that I have up there are what I typically use, but in my mind, there are only a few spells that WOULDN'T work.

cleanse: If you have trouble with cc, up the phil stone to a miracle quickly...if you ABSOLUTELY CAN'T F*&%ING TAKE IT ANYMORE, get a quicksilver sash. its cleanse, but with a lower cd. it also takes away an item slot instead of a (generally) MUUUUUUUCH more important summoner spell.
Rally: ...really...
Heal: heal is for scrubs. Sorry...that's my opinion. it's actually probably a viable spell, but in my mind, this is just annoying early game and flat out stupid to have late game. besides, there's no point in having this when you can't die anyway.
Clarity: it's really only good early game, and since we can solve this in my build really easily, we don't need it.
Promote: YOU'RE THE FREAKIN' TANK!!! KINDA!!! do your job...sheesh...
revive: .........................you don't die.....

Anything else is good, though i'd take ghost, TP, exhaust, or flash before anything else.

Pros:
Sejuani is seriously one of the hardest champs to escape from...BEFORE she has more speed and slows.
You can tank without sacrificing a ton of potential damage
she rides a freakin' hog...come on guys.

Cons:
early game, ranged champs will annoy you to no end.
if your entire team feeds, and you end up in what seems to be more like a 2v5 than a 5v5, this does a lot, but i can almost guarantee you wont win...you'll just be 12 and 5 and very, very angry with your feeding mf and vayne.

If this doesn't work for you, don't thumbs it down. It may sound like I'm just going for rating here, but seriously, there's a couple of other things that could be going wrong. My build may not be to everyone's better playing styles (especially if you jungle, because i'm not a viable jungler for any team i've ever played with.) and even so, there are some team comps that seem to almost be made by satan. (i mean really...who thought of teaming kennen, amumu, ashe, vayne and fiddle anyway!?! that was literally the worst game of my life...i was like 1 and 22...fear the aoe ults people! but i'm off topic, and that wasn't with this build anyway...moving on) This guide is only that: a GUIDE to help you learn how to play sejuani. if someone else's build works better, or if you come up with a better build in general, then use that. And by all means, point it out! I'm not the best player, and i'm not gonna make the absolute best build every time, so if something works better than mine, i'll give it a try. heck, if it works even for me, I'LL POST THAT BUILD HERE! (with credit to its maker...i'm going to write "go see this build" not make a mirror of it and call it my own. I'm not that guy.) Anyway, that's all I can do here. so go on and play some league, and stop trying to figure out if i've lost my mind...unless you found it...then i'd like it back.

Reyn out.