So, you wanna play aurelion sol support and don't know where to start?

You must first know your greatest advantage is time. The longer the game, the more stacks you gain and so the stronger you are. In case you don't know what stacks do I'll explain that in another chapter. All you need to know now is that Stacks = Power.
Taking that in consideration, you need to play SAFE.
Do NOT engage in the early game. Try to get as many stacks from minions without harming your adc's farm too much. Try to fight only when your jungler ganks you.

In the late game, remember. You are still a mage. You die fast.
You must go as late as possible in the team fight, do not be that guy who dies first and then blames everything on his team...it's your fault if you don't position yourself behind your tanks.

In case you do not read abilities...which you should...I'll quickly give you some ideas about what the stacks do most importantly:
- lower execution threshold on e
- higher damage on q
- abilities have more range

Be patient...
Do not use your e everytime it's off cooldown. Aim to get at least 3 stacks with every use.

A cannon minion gives you 3 stacks, so be sure that everytime one dies, it dies in your e. In case you put your e too soon, you can help in the early game by attacking, because you have that golden support item.

Barons and dragons also give stacks, so don't forget to use e when they almost die.

In laning, focus more on the ranged minions rather than the melee minions, because they die sooner and you have higher chances of getting all 3 of them.

In team fights, you are a mage. Weak, but with high damage potential. Let your tanks engage, and then come with w -> R -> E -> Q to help them.

Try not to die first, look at map constantly, be close to your team.