Yuumi as a laner suffers from mana deficiency in the early stages of the game. What's worse, is her poke (Q) doesn't do a lot of damage, is a skill shot, and has a slow shot speed. BTW, your Q shoots straight unless you are mounted on your ally's shoulder and hits the first enemy target. It does slightly more damage if it travels at least 1 second.

Play it safe and farm until you get to at least level 6 where you get your ultimate.

At level 2 get your 'Zoomies' (E) for healing to increase your sustain. Depending on your matchup, you will want to either level your Q first, or evenly split with your E. Unless you have a lane partner, 'You and Me!' are wasted points at this stage.

Hopefully you have been grinding cash and keeping safe. Mid-game is when small team fights start happening as teams start pressuring lanes or going for the dragons. Yuumi's ultimate has a wide blast and roots upon the 3rd hit. Best of all, she can move or cast W or E while it is firing! Like Lucian's ultimate, Yuumi's ultimate continues to fire at the same angle you started while you run like crazy. This is where being on your ally's shoulder (invulnerability! yay!) can be awesome!

Late game Yuumi is mainly a healer and CC'er. Slow people with your Q (Prowling Projectile) and root them with your ultimate (Final Chapter). If you have a main tank, mount them and provide healing/shielding. Or, add extra dps and mount your allies for spot healing here and there.

Thanks for reading my guide on Yuumi. NOTE: This is my first guide. These builds are not very playtested and have mainly been built through theorycrafting and some testing vs A.I.s. I'm sure better guides will follow as people play Yuumi more and find a viable path for her to lane. That being said, I think she is a decent support and give thanks to everyone who built a support guide for Yuumi! Meowsers!