Pantheon's rework allows him now to embody any role. His shield, stun, poke, and occasional slow make him an excellent support for those looking for a non-traditional support champion. Support also allows him to overcome his weakness of lane clearing.

Start with Relic shield, as this will be your main income source and will help you gain stacks on your mortal will more effectively. Taking shield along with your flash will help you stay alive and tank more damage, which will become more important after on. Also, start with your Q which will help you poke enemies in early game. A spear to the enemies blue buff bush to see if the enemy is there will be appreciated by your jungler. Poke your enemies and protect your ADC is your top priority early.
At level 2 you will want to take your W if you believe your ADC will win a team fight or E to play more passively. Then take the other at level 3. These two abilities are the bulk of your support kit. Use your W to stun an enemy and secure a kill for your ADC, I prefer going in with 5 stacks when I use my W so that I can follow up with a Q with mortal will that slows the enemy, giving better kill opportunities. Your E can be used to defend your ally from a strong attack, tank damage during a team fight and even block turret shots. It is important that you communicate with your team that you can tank turret shots, as it can be used to break weak turrets with no minion wave or even allow your ally to attack past turret while you tank.

By Mid Game you will want to have built your Remnant of the Aspect first. This will help you get gold for other items. I then take Ninja Tabis, unless my ADC is complaining about not having a healing support, in which I calm them by taking Redemption. Either way you will want to build your support item first and your Ninja Tabis or Redemption later. Ninja Tabis is going to help you take more damage and Redemption not only supports your team, but will be key in the special move later on.
Your focus now is to secure kills for your ADC or finish off low health targets with your spear. Your Q benefits the most from being leveled up, so level it up first, followed by your W. E is the most important part of your kit but does not benefit much from being leveled up, so fill it last.
You may not see much use out of your R during the laning phase, but communicate with your mid to spam ping if the want your to ult into mid lane to finish off a low health target or turn a close fight.

From here on out your build is focused on tanking damage during team fights. You will want to either build Sunfire Cape or Thornmail in preparation from your ultimate tactic. If your team is split up, you can use your ult to get quickly to a team mate trying to take a tower or save someone on the run. An alt into dragon pit to steal a dragon will win you some major cool points as well. But you will now shine during team fights. Like most of Pantheon support, communication with your team is incredibly important to victory. Tell your team to go into a team fight on your ult, then aim your R for the center of the enemy team (maybe slightly behind as they will try to run). Directly after ulting in, follow up with your E which will last twice as long because of Mortal Will being filled by Ult. Hopefully, your team has followed you into battle, you have captured the enemies attention and they have wasted their spells and attacks on your E. Sunfire Cape and ThornMail will damage them for your team to finish them off. Of course pop your Shield for extra tanking, if your steal alive W a weak enemy and try to get your spear off. There is a high chance you will die, but your team if they followed will win the team fight and be able to secure objectives because of your Legendary tactics.