Hi!
I am a sej enthousiast who has playeed her every role. Sejuani is very much slept on due to her one dimensional playstyle. If you do start playing Sej I very much recomend one-tricking her. She has alot of tricks that are hard to get ahold of at first.

Toplane Sej is very good on some patches and not so good on others. The main perk of playing Sej top is that many people do not know how to lane against her. Sej top is best with a damage based mellee jg like Nocturne.

EARLY GAME:
Early you should try to stay back and farm until at least lvl 2. At level 3 you can be more aggresive and start taking trades. Level 3 if the enemy over steps you can Q-AA-W-AA-E them to take a big chunk of their HP. If they are about 60% hp you should usaly be able to kill with this combo and ignite. If you are going the new item Fimblewinter then back early to buy tear and boots then tp back to lane.

MID GAME: Sej thrives when she is fighting with other champs. try to get you botlane to take mid so that you can split bot for dragons. Do not be afraid to roam. You can easily shove in your wave and walk midlane for a gank. Mid game is when Sej is the strongest.

LATE GAME: The one downside of sej is that she does no DMG late game. Your main job is to be a meat sheild and get objectives. Play for whoever is fed the most. If you do happen to do damage late game due to a gold advantage then you can easily role the other team

RUNES:
Grasp: Grasp is Sejuani's best rune for toplane. You can take either Percision or Insperation as 2nd tree depending on how hard the matchup is for you. Demolish is good in most scenarios, but dont take it if you're gonna be under tower all game.

Glacial: I have not found many situations that glacial is better than grasp. It is very good if you are going to build support top and hope that your Jg will carry you

Electrocute: This build is kind of troll, but if you want to build full ap Sej top go for it.

Items: For mythic items I suggest going Frostfire pretty much every game. You can go rocket belt or evenshroud only in strange builds. Winter's approach and Warmogs are my favorite items on her, but they do not add a ton of team fight assistance. You should try to buy a tear early for Fimbulwinter, then finish it 2nd item. 3rd Item is most often going to be Thornmail. after this you can either build warmogs or randuins, up to you.

The new main build has you building demonic embrace following Frostfire. Demonic adds a substantial amount of dmg without sacrificing much armor. Deadmans and Force of Nature are good for the movement speed and being able to catch out enemies mid-late game.