Hi everyone! This is my first ever guide here on this site, constructive criticism would be nice :D.

Well, anyways, I've been playing LoL for a few months now, and I recently bought Sejuani, much to my delight. I wanted to go with a tank build, so here you go!

With masteries, I go 1-21-8 for the tank build.

In Offence, I get 1 point on Summoner's Wrath to enhance Exhaust.

In Defence, I get 21 points to improve general survivability and speed.

In Utility, I get 8 points mainly for the speed, but the extra mana helps :D.

For runes, I use a lot of flat defence and magic resistance per level to help her with early game defence, and late game survivability.

For items, I normally start off buying a Ruby Crystal, and later building it into a Heart of Gold for some extra early money and HP.
Then you get your Boots, and build a Philosopher's Stone for more money and HP/MP regen.
Here, you finish your Merc Treads, and build your Aegis of the Legion to support your team mates.

Now here's where you can go your own way:
If the team has AP-heavy characters, build the Force of Nature.
Otherwise, go with the Warmog's Armor/Randuin's Omen.
Then follow up with whatever you didn't get. :D
Simple? Yes.

Now, I find that Sejuani's early game is really weak, and since this is a tank build, don't expect a lot of kills :P.

Anyways, I take 1 point in her Q, then her E, then her W.
I normally max out her E first to help cripple your enemy team.
Then, I max out her Q to reduce the cooldown on that, since it's a pretty big cooldown at level 1.
Lastly, I max out her W. Now, her W isn't a bad skill, I just happen to max it last.
If you want more damage output though, take her W earlier.
But I think it's best if you first max out her E to provide the Permafrost that your team needs to secure kills (or even KS :D).

Of course, take her ultimate whenever you can.

Okay, well this is all I'll add for now.

If this turns out to be a good build, I'll add more onto it.

Thanks for taking the time to read my guide! :D