Alright, so here's why I like Sion
Pros:
Tanky as hell even without tank items
Pure burst, bush strats, and duskblade
The ability to make short work of almost any squishy without them knowing it
Engage power.

Cons:
Can be bursted
Reliant on his Q
Useless passive against good players
Weak against mobile champions

The build has all the notes you'll need, I just wanna add that you should experiment with your own style and whatnot just make sure to get the core.
About laning, just use the E+Q combo often, your not charged Q still does good damage so don't get greedy, if you miss the charged Q but land the slow, it's still good enough
Also use the middle bush often to hide your Q, in the first 7 minutes people will generally be confused and not understand what's going on since he's not picked often, then they'd read it so don't use it when you see that they realized how it works
For combos, Sion has 3 main combos
The lane trade : E+Q+W+Reactivate Q+Reactivate W, this is the general trade sequence with Sion
The engage : R+W+Reactivate/hit R+E+Q+Reactivate W+Q, simply put, this is how you engage a fight for your team
The Flank : E a minion+R+W+R+Q+W+Q, very simple, slow them then charge them then use W and Q

Okay so for the runes, you take the domination tree as is, then the 2nd tree is optional, I like resolve with the stuff I put in it because it helps secure turrets and also helps with the HP stacking game but taking something like coup de grace with legend works quite well too

Okay, so as AD Sion you can splitpush like a madman, or teamfight, or secure objectives
Grouping, generally put, is good if you'll seige, engage a fight, or protect an objective, Sion is not good enough at securing objective due to his lack or resistances as AD Sion and the fact that he lacks any form of sustain damage beyond his AA, other than that, just ignore fights, go for turrets, then use the flank combo to re-join the fight

For teamfighting, you've got 3 objectives
Kill the carries, peel for your carries, or engage
Identify your team's weaknesses and strengths, if you have no form of engage you're the engage ball for your team, if you have enough engage and peel then try to flank and instagib someone from the enemy team then reset, wait for duskblade and Q, then repeat, if you lack peel and the enemy has say a zed or a talon, use Q and R to peel for carries and pick off assassins

Back to itemization, just build according to the enemy team, there are items that are useful but I didn't mention them in the top, getting a last whisper item when having to fight a lot of tanks or fighting healers is generally a good idea, in terms of tanky items I listed the ones that are worth looking at but if you've farmed up the right way you should almost 100% of the time go with warmogs as it will allow you to go into the fight, kill someone, get out, then regen your entire healthbar