Hey welcome to my first guide. I'll just note some things for you:

-English isn't my native language so feel free to correct me.
-Criticism is ok! I'll try my best to improve that guide more and more.
-This guide is build on the less is more principle. I'm not a guy who is posting vast text blocks and alot of picture stuff. You should know the basics of playing Sion (which are findable in other guides).

I wasn't able to find any Sion Dominion guide fitting for me. Maybe it'll help others.

I'm using 22 8 0 myself you can also use 21 9 0.

Feel free to use the Runes that you want. The given Runes are an advise on my part.

Good Speels:

-Ghost || good for escape and early push
-Exhaust || good for escape and earning kills
-Ignite || pretty good for making the first kill
-Clarification || good for escape and getting out of 3 hours stun

Maybe not the best choice:
-Flash || maybe good for escape and earning kills but in my view pretty useless in dominion
-Garnision || maybe useful but i don't use it myself
-Heal
-Surge

Things you should not use:
-Reanimate
-Clarity
-Advance
-Clairvoyance

Make sure you rush to the top tower and give your team the first kill by doing your Q. Then do your random def/push stuff.

Usually I end up with about 15-25 kills and 6-10 deads.

Pros:
-Stun / Good for kills and escape
-Nice Damage
-Your ulti let you win almost every 1on1
-If you're doing well 1st/2nd/3rd is yours
-Good for teamwork
-OP lategame

Cons:
-You will be considered as noob
-It doesn't make fun playing him after a while
-A bit sqishy beginning (depends on enemy team configuration)

I hope it will help you to get started with Dominion Sion. Have fun please rate / comment.