when your building a tank your build will almost always be different in order to confiscate for the more ad or ap enemy champs but here is a decent plain build to start some ideas..her base cooldown time is rather insane so throw some CD runes in there for some help then just load up on armor and magic resist item wise your frozen heart reduces attack speed of enemy's which is helpful in team fights while giving you manna and as sejuani you are rather manna hungry then building heal, and force of nature giving magic resist and giving movement speed as an added bonus other then that everything is rather self explanatory
i hope you like it and be sure to rate and comment it please and thankyou:]