Arcane Comet provides extra damage in laning phase to poke out enemies so your ADC doesn't get bullied. If you can hit your Q consistently you can compete quite well with other poke supports.

Manaflow Band is a no-brainer here as Yuumi uses quite a lot of mana with level 1 Prowling Projectile using 50 mana and level 1 E using 80 mana, both abilities costing 10 more with each rank.

CDR is one of the most important stats on Yuumi so Transcendence provides a lot of utility.

Scorch once again provides some extra poke in lane which helps your ADC get through the weakest part of Yuumi's game.

Presence of Mind again helps with mana sustain both in lane and late game in team fights.

Most enemies will have higher max HP than Yuumi as she has one of the lowest health bars in the game so Cut Down gives a bit of extra damage.

STARTING ITEMS
Spellthief's Edge obviously. As Yuumi isn't required necessarily to get out and proc her passive anymore in Health Potions are optional. You might find yourself backing and not having used your Health Potions so if you're confident you won't need them you can just save a little gold and not buy them at the start of the game.


CORE
Moonstone Renewer is busted IMO, I can see the appeal of Shurelya's Battlesong but I think Moonstone offers far too much to not buy every game. As Yuumi doesn't have a heal on her E anymore, her passive doesn't heal for that much and the heals you get from Moonstone are ridiculous.

Boots of Lucidity is a bit of a hot take because a lot of Yuumi players s**t on buying boots and say they're a waste of gold which I can see a case for. However, I personally would rather spend 950 gold for an early 20 Ability Haste and 12 Summoner Spell haste which usually lines up with when you get your R Final Chapter and also usually wanting Heal & Ignite for fights post level 6.

Chemtech Putrifier is just an insanely good stat-stick with the added bonus of grevious wounds which is useful in almost every game.


FLEX
Ardent Censer has no wasted stats and provides nice buffs to ADCs that benefit from attack speed/on hit.

Staff of Flowing Water, essentially the same stats as Ardent Censer and will often buy this instead:
- If your ADC benefits from CDR (almost a mini Navori Quickblades)
- Doesn't need more attack speed because they're hitting the cap already (not super common but eg. Tristana with Q Rapid Fire buff plus attack speed from your E, Zoomies
- If you have an APC like Karthus or Ziggs
- If you have a team that has a lot of AP mages and you think you will be spending a considerable amount of time on them in fights (if your ADC is really unable to do anything because of assassins etc.)

Mikael's Blessing has a wasted 50 magic resist but if your ADC needs a cleanse then they need a cleanse. Alternatively, they can also build Quicksilver Sash but it may slow their build down, or take summoner spell Cleanse, but this is for you to figure out if you need another cleanse for your team.

Knight's Vow this item has a lot of wasted stats however it has a case into a comp where you need more burst survivability for your ADC. I would probably only build this as a last item and build Kindlegem in your last slot until you can upgrade it to the full item as the rest of the components give you nothing. It puts in a surprising amount of work for survivability to help your carry surive just that fraction of a second longer for your next big burst of shield/heals.

Q Prowling Projectile
IN LANE
This is your main (and basically only) source of damage in lane, keeping the enemy poked down is important so your ADC doesn't get bullied out of lane and off CS. You may find yourself running out of mana if you spam it too much and then not have any mana for Zoomies when you need it in crucial moments. To help manage mana and get the most out of it, you can time it when your Feline Friendship, Arcane Comet and Manaflow Band are up.

MID/LATE GAME
When you have finished your Moonstone Renewer hitting your Q will heal. Use this to poke and top up HP as well as healing in fights.

Hitting this ability when you're about to fight/while fighting is important as it gives your best friend extra on-hit damage which can sometimes be the difference between getting a kill or not.


W You and Me!
Pretty straightforward ability, just be careful getting out/switching taxis that you don't get hit by CC or it will go on cooldown and 90% of the time out of lane you will die before you are able to get back in someone.
When leashing you can get inside the jungler to give them some on-hit healing while using empowered Qs to help leash, it's not massive but helps a little bit.

E Zoomies
LANE
In lane it's not sustainable to be using it often however if you're fighting or your ADC is going to get hit by damage/poke that is greater than the shield it's worth using. You just have to hope they aren't an ability magnet so keep your mana in mind when chosing when to shield in lane

MID/LATE GAME
When you finish Moonstone Renewer your E will also heal if your taxi is in combat. You can use E after disengaging if it's still in within the "in combat" threshold and it will give a little heal, which is quite effective against DOTs as you may be able to squeeze in a few extra heals while being at a safe distance and then go back into a fight.

R Final Chapter
Huge power spike, always want to be on your best friend when you use this ability as it empowers the heal quite a bit. Pretty straight forward ability, use in team fights to give AOE heals to your team and slow enemy team.

STARTING ITEMS
Don't buy Health Potions first buy as you often don't need them.


LEASHING
You can hit 2 Qs on the camp while leashing, throw the first one in a circle to get empowered Prowling Projectile and SIT ON YOUR JUNGLER for on-hit heals until your ADC is ready to leave. Jump back on your ADC and throw another Q as you're walking away to hit a 2nd empowered Q.


TRADING IN LANE
Time your Q when Feline Friendship, Arcane Comet, Manaflow Band and Spellthief's Edge are up to get the most out of Prowling Projectile.
If you miss your Q, you can get out and auto attack to proc Feline Friendship and jump back in.


MYTHIC BUILD PATH
After rushing Bandleglass Mirror, Kindlegem is not the most effective use of gold and doesn't make you much stronger (10% CDR, wasted 200 health) until you finish Moonstone Renewer.
I like to get Boots of Lucidity before getting Kindlegem as the health from Kindlegem reduces your cut down damage (not completely necessary, just min-maxing to the extreme here). Geting Boots of Lucidity usually lines up with the time you have your R Final Chapter and are looking to potentially fight with summoner spells Heal & Ignite. It delays finishing your Moonstone Renewer a bit but having R and summoner spells available when you need them should compensate for this.


SPELLTHIEFS GOLD
Stacking Spellthief's Edge when it is up is important to get your Moonstone Renewer as soon as possible (as well as your wards from Frostfang. Obviously using your Q Prowling Projectile in lane is the main way to stack it up, but autoing towers is also important when the enemy is not in lane or a threat when at their tower.
If the enemy bot lane is not in lane, you can have your ADC start backing in the lane bush, auto the tower 3 times for your Spellthief's Edge stacks, and jump in at the end of their back to go back to base with them.


MOONSTONE HEALING
Using your Q Prowling Projectile and E Zoomies when your teammates are not actively fighting but are still "in combat" will give them a heal from Moonstone Renewer.
If they get hit by small damage poke, it's often better to let them take the hit then press E afterwards to heal up with a net gain of health, rather than just shield the actual poke.
This works well for abilities that have DOT like poison or burn as it keeps your teammates "in combat" for longer, and you may even be able to squeeze in a few extra heals.


FLEX ITEMS
Ardent Censer if your carry benefits from attack speed/on-hit
Staff of Flowing Water if your ADC is at attack speed cap/benefits more from CDR/you have an APC instead of an ADC/your ADC is getting blown up and you need to sit in other people (AP mages) in late game team fights
Mikael's Blessing if you need a cleanse
Knight's Vow if you're feeling spicy, can help against burst assassins that are blowing your ADC up or just high damage in general