playing sion is fairly easy, but building wrong can send you all over the shop.
it might sound easy but alot of people build him stupidly.
starting off with the sheen is a generaly great idea to add that AP onto the edge of his AD parts, making him stronger in both areas. same for the trinity force, the rest of the items are main AD items and by your 5th item you should be destroying the game already with AD, so the gunblade adds abit more AP just for kicks ;)

unlocking all 3 of his abilities by level 3 is essential for early comboing.
his Q is a basic stun that also does damage. his W is a shield, that can also explode after a short period of time and cause damage to untargetted enemies around him, at about level 3 in this ability it starts to do some real damage. His E is a passive that can be activated and de-activated. when activated it increases his attack damage, but also costs health to attack, but only 10 HP, but on the bright side, after every kill you get with the passive active you gain total health based on what level the ability is.
His Ultimate (R) is a Lifesteal, Attack Speed combo, increasing his lifesteal by 50/75/100% based on level, and healng nearby enemies per attack for 25/37/7/50% damage dealt also per level.

comboing with sion is quite easy. Just start up the shield and wait till it is ready to explode, then stun your enemy and auto attack them with your E passive active, and as the shield explodes it can cause amazing damage at an early stage in the game. flash is used in a more advanced way. for example waiting in a bush with your shield active and then flasing on your enemy and exploding them with the shield and auto attacking, and if they are at urge of escape you have your stun ready to catch back up and grab the kill and ignite is there just incase of that tricky escapee you can just ignite, sounds like that wouldnt happen often but it does help alot for early game laners.
Using that in a team fight is good for he has relevantly quick cooldowns compared to how fast you kill someone singled out in the fight, but then the ulti is used in teamfights to keep your team in action for longer with its healing aura

It would be great if you could rate this guide on how much it helped you and your outcoming results, tahnk you for reading this and have fun summoners!