I will be discussing the way succeed most when playing Sion with topics on Pros and Cons, Skills, Spells, Masteries, Runes and of course my item build.

Pros

Tons of health with no items (if you last hit enough)
Very high attack damage with minimal items
On top of having lots of health you have a shield to make you even more tough
Snowball champ - your health will never stop increasing on extremely long games due to

Enrage, with Atma's Impaler, this means your AD will also constantly increase
Easy to use stun
Ult gives amazing life steal


Cons

Not extremely strong early-mid game unless fed (1-18 minutes)
Extremely susceptible to blinds
Mana starved (until you complete manamune)
Slow attack speed

Skills

Really nothing to it, you get a chance to take a little less damage. Not the best passive but you make up for it with other abilities.


Effective and easy to use stun, also helpful for last hitting runner because it does a decent amount of damage


Very useful ability for almost anything. Use while jungling and explode it after it takes some damage for you, use it for saving your @$$ when your at 10 health and get ignited, or charge it up early in a brush and use with Cryptic Gaze for an excellent gank/harass.


If im the team decides not to try and gank at the start of the game i always grab this first to maximize your health bonus, otherwise ill take stun to help the team focus a target when no one has enough abilities.


This is your ult, it is extremely good for team fights because it heals your allies near you as you damage opponents. The attack speed is always great because of sions low attack speed and the lack of it in this build. I usually pop this around 5 seconds into a team fight, because it has a 20 second duration, and if i am 1v1-ing i will pop this once my health is around 50% or if i need to burst down an enemy before they can escape. I like to use Death's Caress before i use this to minimize damage done to you and give yourself a chance to life steal without losing any health - very effective.
Cooldown is fairly short so don't be afraid to use it even if you think you might survive.

Summoner Spells

What I Use...

A MUST HAVE
In 3v3 always i always go with ghost because it allows you to beast across the maps and get to those team fights when you are most needed.

In 5v5 i will take teleport because it essentially provides the same thing as ghost but more effectively on a larger map. If you like ghost to juke people out and escaping from dire situations then take it instead. It just depends on how your feeling.


Extremely good for disabling the carry, and if used properly with with Death's Caress and Cannibalism, you can maximize the amount of health you regain because you will not be taking very much damage (1v1 or 1v2, anything other than that and its unlikely you had a chance in the first place.)

Other viable options


Good for getting the last bit of life on someone fleeing the battle, i find exhaust useful in more situations so this one is up to you.


Great for getting out of ganks and to keep the focus off of you


Amazingly short cooldown and unbelievably useful. I usually let my friends grab this one but if no one has it can be worth taking


Not my favourite but a 500 heal at 18 with your ult can really increase survivability. take this if your low level and having trouble with health.

What Not You Use

Would only be useful early game if you spam all your abilities trying to harass, i just take a mana pot and once you get Tear of the Goddess you wont have mana issues (unless you are reckless with your abilities) Not recomended.


Great escape/initiate spell but ghost scaled well with the speed of Phantom Dancer and Boots of Swiftness. Not as effective as ghost so don't get this.


You are not going to be jungling as your main source of gold/exp. Only as often as every other champ who needs the occasional buff/gold, your Enrage is not good for sustaining health while jungling and you will just mana starve yourself quickly, making you almost helpless to ganks and useless in fights.

Masteries
Pretty Standard physical DPS 21/0/9, getting improved Ghost / Teleport and Exhaust. If your using other summoner spells you can change around the points a bit but make sure to get Deadliness , Alacrity , Sunder , Brute Force and Lethality in physical. In utility you can choose Meditation or Greed . I like Meditation because it helps with early game mana problems, if you dont find this useful take Greed or even Utility Mastery .

Runes
Greater Mark of Desolation
You will hit harder, nuff said.
Greater Quintessence of Desolation


Good for capping crit chance and early game critting

What to grab first?

For the good early game critting. At level 3 you can crit as much as 223, without only armor pen runes, that's why the crit runes are important for early and help keep your crit close to 90% late game.
Health pot because you will be losing health every attack and being harassed, mana pot in case you over abuse your spells.
OR


The speed can help you get a few more hits on your laning opponent when you stun and is nice to keep their harassing to a minimum.

Core Build


This is part of the core build but I advise getting it a bit later when your health is really built up, or if you need the armor.