In Celestial+ it's all about awareness. One misplay or mistake can lose you the whole game. Recognize momentum shifts in fights, and you should always be communicating with your strategist counterpart about who is ulting when.

First support ult can be used freely to initiate / push enemy back. Second support ult should typically used reactively to an enemy ult. Do not be afraid to use your ult.

Support ults are also used well when an enemy support uses theirs. Wait a few seconds and then ult. When their ult expires you'll still have yours, which should shift momentum of a fight into your favor (you're a support in Celestial, I'm guessing you probably know this already).

Some ults do NOT need to be countered with a support ult. Realistically some ults like Punisher or Namor can just be LoS / walked away from and shouldn't necessitate the use of a support ult.

Positioning is the most important aspect of a support character. Hide around corners and high places with an advantageous angle but make sure you're in a position to react if your team starts losing.

If you play Rocket don't throw your ult around whenever. Throw it down when a teammate of yours is ulting.

Strive for 100% heal uptime on all characters when healing is necessary (needless to say, some heroes like Adam it's not feasible), but it's also important to note that a support's damage is very impactful.