everyone has a different ultra combo input, check the command list to show you what theirs is. You can only perform the ultra when your opponent reaches "danger" on their health bar, but you cant just do the ultra combo, it has to be linked within a combo, so you can do a combo opener then the ultra right after.
Even if you are in the middle of a big combo and their health reaches danger zones while you are comboing them, you can finish them in the same combo with the ultra
you can also do stage ultras which are harder, you need to be at a certain spot in the stages to do them (only 5 stages have stage ultras) but they are cool, they are like stage fatalities from MK
TJ combos stage your opponent gets hit by the wrecking ball, Hisako's stage your opponent gets pulled under by ghost hands, Aria's stage you can knock your opponent out the window to their explosive doom, Aganos' stage you can turn your opponent into a rock statue, and in Kan-Ra's stage your opponent gets hit by multiple stage items then gets sucked through a portal
nice! yeah I figured if im only going to get it for one game i'd feel less guilty about it if I got it for free

t: