Engaging early as pantheon is a 100% must do. His late game is weak beyond belief, so your main job is to feed your ADC.

Look for level 1 or Level 2 engage onto the enemy ADC (or support in special cases. See counters + synergies) Make sure to ping to ensure your ADC is on board. This can sometimes mean skipping the leash and waiting in the lane bushes for the enemies to show up, then surprise them by jumping out of the bush with your W and having your ADC follow up. Force a flash, then disengage.

Flash advantage is something that is key in 99.99% of early pantheon support kills. Your first engage should not be to KILL the enemy laner, rather force a flash so you have one and they don't.

After you force a flash at level 1 or level 2, the enemy ADC will either come back to farm or back. 90% of the time they will NOT want to miss out on early game farm and risk giving you a level advantage on them which will likely lead to an early kill too.

If the enemy ADC chooses to farm Flashless, look for a Flash + W combo. They will not have flash to escape it, and they will be weakened enough by your first engage that they will die to your ally ADC 99% of the time. This gives your ally ADC an early BF sword, which will lead into a snowball, especially if your ADC scales well like Jhin or Jinx.

When engaging at any time in the game, ALWAYS engage with a Passive-primed W. This will cause your next auto attack to both refund you 3 stacks of your passive AND proc Press the Attack. After your follow up auto, immediately Q the enemy and escape with E a millisecond after. If you did your math right, you will walk away from the engage with a primed passive.

Since you used your W as your engage tool when you walk away from the engage it should be nearing the end of its cooldown. If you have the flash advantage, this will lead right into a re-engage and a kill.