Don't just sit on your ADC especially in laning phase. To maximize gold poke the ADC with your autos while they are in their own auto animation then hop back on your ADC before you're hit giving them a shield if your Aery has come back. That being said you can take a few hits. Minimize the use of your E on yourself as it already has a huge mana cost when you could just stay on your ADC and use your refillable pots.
Lastly spam living hell out of your wards in key areas such as tri-bush, dragon, and river bush/near scuttle. There is no reason you shouldn't have a 35 vision score at the very least.
As normal try to not steal kills but it is better to secure a kill than possibly lose out on it. If you're fed as Yuumi you'll be ahead in items allowing your ADC to be more aggressive due to your buffed cool downs, healing, and damage which gives them more kills in the end.

In team fights you will typically want to stick on your tanky engage. Do make sure to hop around to teammates who need the healing (the Yi or some tank with two thirds of their hp can wait while you heal your poor Syndra who's missing one fifth while being engaged). As Yuumi you will be juggling targets similar to Medic(TF2) or Mercy(Overwatch).
Just be very careful of anyone with CC as they can ruin your day mid flight putting it on a 5 second cool down leaving your squishy self only able to dance with the hope they let you live.

For the love of god, play Yuumi to have fun. While you can feed your team kills they WILL leave you to die so learn when to hop off your teammates who are about to die after staying your welcome. (note your ADC will appreciate the longer you stay even if you die at times early on letting you be more generous with how soon you leave later on.)

This guide was written to help out my aspiring Pro Yuumi friends and as such is written in a lax/rushed manner. I am going to regret staying up till 6am when I go into work tomorrow.