Thebausffs

Guide by the greatest Sion player who inspired my item selection for this guide. You can also watch his videos and streams to learn how he plays more extensively.
Below is an essential guide for Sion players, relevant part of the guide is from 10:47 to the end. In the description there is a spreadsheet which explains how to deal with each matchup, although some advice might be outdated.

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Additionally there is this other guide which is equally important, but less specific to Sion.

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Basics

One of the most important things to remember is to have a consistently high CS rate so you can be useful. Try not to miss any CS or plates in laning, proxy, take enemy jungle and towers, get Heartsteel and Grasp of the Undying stacks. Don't be afraid of dying, but still learn from your mistakes so you can die for a better reason next time. Being aggressive is the only way to improve.


Deciding which build to play:

Tank or AD: This will depend on if your team needs a tank or more damage, as well as how much anti-tank the enemies may have. The spreadsheet in the description of Thebausffs' guide linked above shows which build is better for which matchups. If you're unsure, tank is generally a better pick than AD. AD is not in a great place at the moment.

[*] Try to pick up as much CS as possible for the first 2 waves while shielding to take less damage, but you will likely take a lot of damage which is okay.
[*] Walk up to 3rd wave to Q and die for the wave, while also dealing damage to the opponent.
[*] Afterwards, walk back to lane and since the opponent should recall due to your health and item advantage, push minions quickly into his tower with Minion Demateralizer, using one on each type of minion. If they don't recall you can kill them.
[*] Once you push minions into their tower you can proxy (farming behind tier 1 enemy tower or preferably tier 2) until you die and try getting executed by the tower if someone comes or you run out of mana
[*] Proxying denies the enemy's reset as they'll need to stay to catch the next wave. Therefore, if you proxy and then die or reset, you will have delayed their reset enough that you'll be back in time to get a plating while they're coming back to lane.
[*] After level 7 you always want to be proxying since it'll give you time to get back to lane.
[*] From around 14-25 mins into the game you can go top or bot, preferring the one that has no teammates and where there are more towers to take.
[*] In midgame, if the enemy tower in the lane you're in is defended, threaten roaming after you proxy. If it's not defended, take the tower.
[*] Only teamfight for baron or 3rd/4th dragon and only if it's convenient.
[*] Flank for teamfights
[*] Play safe during late game and look for kills
[*] Don't use speed boost in passive if you are already in melee range
[*] If you use Q while standing somewhere the enemy has no vision, they won't see the outline. Try standing on the edge of a bush to have max range.
[*] If you press Q right after pressing R your camera won't be moved, making point-blank ults more comfortable.
[*] Keep in mind that you dying gives far less gold the more you die, so you can be more aggressive when you are feeding.

Core (Tank)

Take this every game unless you need a situational item first, in which case it may be better to skip Heartsteel all together, as it's important to start stacking it early. You should always look to get more stacks but it's not worth it to die for just for stacks.

This is not an item you have to change your playstyle around much, aside from auto attacking more, which you already should be doing when you have the chance. If you're always dying before you can even do anything, especially if several enemies target you simultaneously, it may be better to take Gargoyle Stoneplate before Titanic.

Similar to Zhonya's Hourglass, you should wait for enemies to start directing their cooldowns on you before you use the active of this item, to absorb the most damage possible and take their important abilities away.

Pretty straight forward item. Just don't use Gargoyle Stoneplate and probably not Soul Furnace while the Lifeline shield is already up.

Consider spacing out your abilities more to make use of damage over time. This is not a great item, but better than taking a situational item you don't need.

Core (AD)

Essential funny item. Saving the active for during your passive can be wise to get a kill.

Movement speed and lethality choice, can be taken before or after Lord Dominik's Regards. You should build it after if you need anti-tank. Saving the active for during your passive can be wise to get a kill.

You can just build Last Whisper for anti-tank and finish the item as early or late as you want, but you should build this item fully before Infinity Edge.

Can be built before or after Lord Dominik's Regards. You should build it before if you don't need anti-tank.

Once you have 40% crit, buy this. If you don't have space for it because you bought a situational item, you can replace boots or Hullbreaker for it if you'd like, but it's not always the right choice.

Situational (Tank)

You should take this frequently. Make liberal use of aura damage when you can. Aura size increases with champion size so keep it in mind when you increase size due to Heartsteel, Gargoyle Stoneplate or Elixir of Iron. The main decider for taking this item should be if you need Armor, usually when their top in addition to either their jungle or mid does lots of AD, and it's usually logical as a second item.

Hullbreaker is fantastic for Sion, and it lets you take towers, before you have this item you shouldn't bother attacking towers after proccing demolish unless minions will take a plating without you, since you need to be near a plating when it falls to get gold. The minion buff is a great part of this item so make sure to use it. It's usually logical to take this a second item. If you're playing tank and below Diamond 2, you may want to take this every game to solo carry. Otherwise, you should take this item if splitpushing is going well for you and you won't need to teamfight often.

Don't take this too often, but it's good when auto attack heavy enemies are posing a big problem to you. Make sure to use the aura when you can, either for yourself or to help teammates. Warden's Mail is great to take even before mythic and even if you don't plan to evolve it into Frozen Heart, as it provides most defensive capabilities of Frozen Heart for much cheaper.

Similarly to Elixir of Wrath if you're AD, you don't necessarily need to only take this when you have a full build, you can also take it when you need a sudden powerspike to end the game or prevent your team from losing.

If you really need antiheal and this item will work on your target (they have to auto attack you to proc Thornmail). Don't take this too often though.

If their top, mid and jungle are all AP

Vs high CC

Vs lots of auto attackers

Great boots during your passive and for general movement. Take this more frequently than other boots.

Situational (AD)

Take if there's 2 enemies with frequent shields

Take about every game, when it's convenient

If you just need more damage

RESOLVE

Grasp of the Undying is an excellent rune for inting Sion since it helps you increase your health throughout the game so your powerspike will hit that much harder. Since playing this build usually makes games last much longer, since how you usually win is only by surgically removing their nexus via splitpushing, this will help you profit off the extra time with infinite stacking potential. The extra damage that it deals means you can trade whenever you can use it and usually win! Actually you should have W up to trade, but against tanks you only need grasp to trade. Inting Sion doesn't care much for healing but he doesn't mind it, so Spirit Visage helps you gain extra healing from this rune.

Demolish has always been and always will be what makes inting Sion good. Once you hit the late game, this makes towers melt before you. There isn't really much more to say.

Similar purpose to Grasp of the Undying in the sense that it's abusing the fact that you're making games longer by playing this build. Makes you even tankier and all it takes is waiting 12 minutes.

ALSO abusing long games to become tankier. Just have to make sure you CS a lot which you should be doing anyway. A useful way to CS is to R across the lane, and you'll almost always be oneshotting the minions.

INSPIRATION

Simple way for CSing to be easier despite your tankiness. Using 2 on siege minion s and 1 on a melee minion is a good choice.

Great for movement speed and kill pressure since your E and Q can both proc it.

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