I'll try to make this a simple guide.

Okay so besides doing some damage your mission is to get $$$$ from your Spellthief and gain maximum mana from Manaflow Band. And to sustain both yourself and your ADC of course.
Some keypoints:

• Don't spam Q's and E's like a madman. WAIT TIL THE COOLDOWN FOR MANAFLOW BAND HAS REFRESHED. That way you'll get the most out of your abilities.
• Only poke with E if you're sure you won't need to use it right after that cause the cooldown isn't short
• Only heal your ally WITHOUT LANDING A Q FIRST when he is on the verge of death. Always try to heal your ADC right after landing a Q because the heal will be much stronger and you won't lose any health.

Now that you have your R you'll become a global threat.

• Keep an eye on your teammates in case they engage on a fight, to turn the tides of it. Enemies normally get overconfident and don't see your R coming.
• IF YOUR ATHENE'S UNHOLY GRAIL HAS CHARGES, SAVE THEM TO USE THEM ON YOUR R RATHER THAN ON A SINGLE TARGET HEAL

Now things will get wild and you should have your Stopwatch / Zhonya's ready cause in teamfights you will ALWAYS be the easiest target to kill, and also (if you play well) the team's carry.

• Try to get the enemy to use R's and abilities on you to right before use your Stopwatch and leave them exposed.
• IF YOUR ATHENE'S UNHOLY GRAIL HAS CHARGES, SAVE THEM TO USE THEM ON YOUR R RATHER THAN ON A SINGLE TARGET HEAL. This is a big deal
• Spam W right after using your Q. At 40%CDR the cooldown will be ridiculous (like 1sec or less) and thanks to this you'll be able to grant "Rejuvenation" to more than one ally WITH JUST ONE Q.