Start with a defense or support item that plays to morg's strengths. Once you have the defense and support item, you can pretty much go crazy with AP and magic pen. I love Liandry's Torment for late game.
Morgana plays a dirty support. She's powerful enough to hold lanes alone and can almost be a backup carry in the bottom lane, but don't gold-starve your squishier carry friends that can finish kills in seconds. Morg is a poke and sustain champion and just doesn't burst like a true carry until the very late game. As you improve, she can basically stop ganks in their tracks.
#Detailed Outline (I'm an oxymoron)
Support morgana's pivotal role is lane control. Late game, you'll often find yourself wandering into lanes to clear out minion rushes while the rest of the team is holding mid.
- When players begin wandering, controlling unoccupied lanes becomes very important, so Zz'rot is a helpful option. It lets you leave lanes unattended so you can assist other lanes.
- Near teammates in teamfights, land QW and have Q/E ready again to prevent ganks.
- Zhonya's active and R is a powerful combo but hard to time. Even if your timing is poor, the stats on Zhonya's are a huge asset.
- In the early game, Frost Queen's Claim is a good support item to build because it helps recover gold given to your carry , the ability lets you run from ganks, and it keeps your mana/cooldown healthy.
- I enjoy using Athene's Unholy Grail instead. It makes E heal friendly champs after I've bruised the hostile champs a little. The CDR makes R recharge insanely fast.
Morgana supports by being able to keep damage away from her team (and herself naturally), and dishing out tons of damage. Played well, she supports by:
- clearing rushes of 30+ minions at top, in only 5 - 15 seconds with W, and running back to mid to CC.
- Landing QW and Incinerate while your carry is autoattacking a Teemo that hasn't had a chance to CS for the last 4 minutes.