I only use Sion as a ap champ. He have a great "W" "Q" "W" combo. Use your "W" and wait to its finished use you"Q" to stun and then realese your "W". It deals a lot of damege and the coldown is`nt so long so you can use often (But be cautious with your mana).

The Sheen and Lich Bane is great for Sion beacuse it gives you lot of AP and when you hit it gives mutch AD. Morello´s Evil Tome helps you late game buacuse it gives you Coldown Reduction so you can use your combo more often.

I choose Flahs and Clarity to help me early game, but Flash can you allways use. You can also use Ghost or Exhaust

I farm with "W" and then last hit the minions with "E" or "ULTY" if there´s a big group of them.