I'm TQNSHI a no-life Yuumi main with 300k points. For those of you who care about rank; I don't play ranked so highest I've gotten is silver, however, I've had players in plat and dia compliment me if that's worth anything. So I'd say take my guide with a grain of salt, as you should any other guide. I'm no expert lol, I only speak from experience.

Passive - Bop 'n' Block
Your passive gives you mana and a shield when you auto attack an enemy champion. Use this whenever you can but watch out for CC, it's your worst enemy.

Q - Prowling Projectile
Your Q is a skillshot when off an ally which slows enemies once it travels for 1 second plus some extra dmg. When you're on an ally you can controll it with your mouse, if it travels for 1 second it slows enemies plus som extra dmg. You'll want to learn how to curve this, the duration is 2 seconds however so it can be difficult but hang in there. This ability scales with AP.

W - You and Me!
Your W makes you dash to an ally and attach to them, this makes you untargetable (except for turret, it can still hit you). Abilites can still hit you during the dash though so watch out for hooks. CC (except silences) puts this ability in CD so don't get hit. You also have to be attached to your allies for a certain amount of time before you get off because that will also trigger its CD. This time frame is shown underneath the W.

When you're attached you give your ally some adaptive dmg and they give you some adaptive dmg.

E - Zoomies
Your E is a healing ability which scales with AP, it also gives movement and attack speed which makes Yuumi a good fit for any ADC. This ability costs a lot of mana which means you can't spam it, be mindful of how you use it.

R - Final Chapter
Final Chapter is a wide ability that shoots seven waves of magic dmg, enemy champions that get hit by three waves are rooted. You can dash between champions and use E while you're casting R.

Sorcery
Comet is really only useful early if you can hit your Q's, adding scorch to this makes it deal more dmg. Aery is better late because it helps give your team some shields when you switch between allies during fights. Aery also provides some poking dmg. Why you're not going Phase Rush should be obvious, you can't hit the enemy with 3 seperate attacks plus Yuumi doesn't benefit from movement speed.

Manaflow band is the only useful one here, it increases your mana which Yuumi really needs considering the cost of her E.

Transendence and Absolute Focus are both pretty good imo. Transendence gives you ability haste which means you can pump out your E's and your R faster. Absolute Focus is nice because it increases your AP dmg when you're above 70% HP and since you'll almost always have above 70% HP it's a nice AP boost (all your abilites scale with AP).

Gathering storm gives you increased AP over the course of the game so it's great late game. I'd only ever go scorch if I know I can poke a lot early while also getting the mana from my passive, it's more early game and even then I wouldn't use it on Yuumi.

Inspiration
The only keystone runes that could have any use here are Glacial Augment and First Strike. Even then the use is pretty small, Glacial only procs with your ult. First Strike scales with the dmg you do and the only time you'll proc this rune is when you Q which doesn't do a whole lot of dmg.

You have no use of any of the runes in the first row... unless you wanna sell boots for extra gold.

Biscuit Delivery is the most useful rune in this row imo, it helps if you have mana difficulty and even if you get through early without using the cookies they can be sold or used later during extended fights. Future's Market can also be useful since it helps you get ahead by going in debt (uh-oh) when buying items.

Cosmic Insight provides ability haste and item hate which is really nice since it can save your carry in the nick of time. Time Warp Tonic can also be helpful if you have cookies since you gain mana faster, it's only ever helpful early game and since you have your passive I'd opt to take Cosmic Insight instead.

Resolve
You could try Resolve as your primary tree since you'll always be in range to proc Guardian.

Font of Life is the only useful rune in this row. It procs with your empowered Q and R which gives some nice healing.

You could take bone plating if you for some reason think you're going to be off your carry enough for you to get caught, it might help you survive until someone comes but you're Yuumi so probably not...

Revitalize is really nice, it provides 5% extra healing from your E and if the target is below 40% it provides 10% extra healing.

Precision
This is not an AD Yuumi guide so I'll only talk about Presence of Mind and Cut Down. Presence of Mind gives you increased mana regen on takedowns and when you hit enemies, very useful if you know your team will do good. Cut Down makes you deal more dmg to enemies with more max HP than you and they will definetly have more max HP than you.

Domination
If you want to troll I'm not stopping you, in fact I encourage it as long as it's in normals!

Dark Harvest is the only useful key rune, if you poke a lot you can easily get stacks. The DH stacks increase your AP which helps you deal more DMG and heal more.

Sudden Impact DOES NOT work on Yuumi :( the magic penetration would be nice. Cheap Shot works nice since your Q and R impairs enemy movement.

Since you won't ward as much as other supports (explaiend below) take Eyeball Collection instead. It gives you AP for every champ you get a takedown on (including assists) until 10 stacks which is max stacks.

Ultimate Hunter is the best one since you want to lower the CD on your ult as much as possible.

The thing about Yuumi is that unlike other supports your team needs to play around you. Your team needs to understand how Yuumi works to unlock her true potential, like you can be the best Yuumi player but if your team doesn't know how you work it doesn't matter.

Your job as a support is warding but I have much lower vision score on Yuumi than any other champ, why? Because my team doesn't allow me to ward the jungle, you want to be attached to the carry as much as possible but if they're not going in the jungle you'll easily get caught because everyone depends on you to ward it. Sure, you could try to ward the jungle by your own but you can also very well get caught and die. This shouldn't be a problem the higher rank you are but it's worth to keep in mind that warding as Yuumi is hard. Speaking of wards, ward drake/baron/rift with a control ward to deny enemy vision and to secure that objective. Fighting over vision in the river bush can be very costly and oracle is more than enough for that.

Most players also think Yuumi is useless and will tell you to play something else, I may be biased but I beg to differ. Yuumi provides a **** ton of heals, maybe not Soraka lvl but instead you don't get perma targeted by an assassin like you would be if you played Soraka. You also give roots and slows and if your team pays attention to that you can help them a ton. Even just hitting your Q as you're running away can be a life saver!

Early game you want to poke the enemy ADC with your Q as much as you can, you also want to AA the support or ADC (whichever is closer and safer to AA) to have the mana to poke. You also want enough mana to get out of hairy situations for example a gank or an engage support going in. Be extra mindful when you're against the threats displayed above.

Your E costs a lot so be careful of how you use it against heavy poke.

Make sure the river is warded so you don't get surprised ganked, if you're agaisnt a Kayn keep your eyes on the wall. Also keep your eyes on the map, since you don't have to be constantly mindful of where you're standing when you play Yuumi you can check the map more often. Map awerness is really useful!

It is time to sit atop your carry! Keeping them alive and well is going to help your team a ton. But don't forget to also help your other teammates, it's a team game and you won't always be able to 2v5. Watch out for CC when dashing between allies or gettning off to proc your passive. Remember to focus on objectives and keeping the jungle warded to the best of your abilty, you'll do great!