Try and stay attached while your passive is on CD, the quickly hop off and auto to proc it and hop back on to your adc. Constantly use your Q to poke down the enemy even if they are behind minions to zone them.

Don't back until you have at least 800 gold so you can buy Frostfang and dark seal in one back then tp with your adc back to lane.

Once you hit Lv 6 you have a lot more options as your W CD is halved from 10 to 5 seconds and your ult is great for getting kills in lane.

During mid game you become a lot more powerful. If lane went well you should have some stacks on dark seal as well as much better heals and damage with Your Grail and Censor as well as gathering storm.

During team fights try to bounce between team mates healing them. Try and also use your passive right before the fight as the shield can really help your carries tank damage. Try and attach to whoever engages the fight to keep them alive with your passive and E, and ult off of them to try and lock down the enemy team.

Late game Yuumi is a power house. Her shield and heals are huge and her Q damage and slow really add up. Mostly follow the same as mid game but you're a lot stronger. Also Yuumi can reach full build really fast as it's easy to get assists on her.