Basic stuff:

R casts from a bit farther position than starting the cast; usually R during Q dash and break the stun with W. In a typical build you want to save a support item charge for the cannon minions, but this build is all about harassing laners. You can keep lanes alive with W. Q knocks champions back but pulls minions closer. Make sure no enemy is within your immediate size when casting Q to escape. W pushes minions and gathers them, which can disrupt champion pathing or cause them to take a turret hit. Landing R prevents passive/E stacks for a while. Judge the stun bar and enemy cast/attack animations to disrupt or stunlock for longer.

The main idea is that Sejuani is like a longer range Leona. The minimum level is 3, and power spikes relative to enemies are 4, 5, and 6. Most people in bot and lower elos don't expect sejuani R, especially the range after Q, let alone the default combo.

You can cheese top fb from the farthest bush, leveling E after they've engaged