After leashing or going straight to lane, just be around your ADC to collect XP and the extra gold from Ancient Coin. Harass the Opposing ADC with star call and heal your ADC if they are under about 75% health (If you heal too much the team can focus you). Be sure to ward and ask your ADC to ward as well. If being ganked, use Equinox to silence the jungler or support as they are the ones who usually have the most cc. After getting 800 gold IMMEDIATELY get a sight-stone (warding wins games) also switch for the red trinket. After getting enough gold, get the combination item Eye of the Oasis. Also, Don't be afraid to use your Ult as it is very strong and can be the difference in fights.By this time you should be close to level nine, and you should get the Oracle Alteration so you can clear more wards.
The next item you should get is your boots of mobility, this will help you get around and heal your carries more often. After that, build Blasting Wand into Rod of Ages, this will help a lot later in the game. Next you should build Locket of the Iron Solari as it further shields carries from damage.
Then, depending on the situation get something like Zeke's and then Warmog's. Warmog's allows you to spam your heal without worrying about your health. Zeke's can be useful for both the AP and the Armor with will probably be used on your ADC. Finally, you should get Alacrity on your boots and you will have completed the build.
PS- Don't forget to ward.
Pros / Cons
She is a pretty easy champ
Low mana cost
Has solid skins
Low solo mobility
Low damage output
Lith this build she loses a bit of utility