sejuani is a great champion all around. Lots of Cc, slows, snares, and stuns. but many do not know that sejuani (while also being good at jungling/tanking) is a great ap off tank, in this build i try to show off her true power while also making her able to endure almost anything!

for her runes i have chosen x9 greater marks of resilience, x9 greater glyphs of vitality,x9 greater seals of warding, Greater quintessence of vigor, greater quintessence of resilience,
and greater quintessence of sheilding to provide armor, hp, and magic resistance.

for masteries i went 9/21/0 to provide more resilience to damage and add a little more ap for l8er on in game. i chose according to the benefits of being able to come in and save a team from completely wiping and to also be able initiate and still do enough damage to push back the enemy so your team can have breathing room and/or secure kills.

for items i chose to start with with a regrowth pendant to be able to stay in lane longer so that you wont need to go back that often. the philosopher's stone is to start building your shurelyia's reverie. The rod of ages is for early/mid game (depends if you actually get fed or what not early game) ap and hp. plus the unique passive is great for early/mid game (before the ganking and roaming starts) laning and extra sustain to survive a gank. mercury treads for movement and to lower the stun and slows of your enemies (if by any chance you don't like merc treads then try Ionians for more cd reduction that's what i usually do :) ) once u get your heart of gold finish your randuins as fast as possible for more sustain armor and hp regen.
finally the ap items by this time you will be 15-18 once u build your ap items(archangles and rabadins) you will top out at almost 500 ap around 110 physical damage tons of armor and magic resist and about 3k hp in other words your somewhat hard to kill and people will hate u. :)

for summoner spells you should get teleport/heal or tele/ghost or tele/flash to save your self early AND late game. or to be able to catch up to a team fight when its just begun.

in this build you wont find much problems besides the fact that you will be trotting until you get boots (which could lead to your demise if you totally fail) but other than that your pretty much set for anything. you will be loved if you conquer and you will get added by tons of people :D