How do you Yuumi? :3
From Level 1 to 5 I usually upgrade Q first then E once level two. By three, four and five I try maxing my Q and W so it's more effective to whoever I'm attached. I'm always told during rank to carry exhaust and ignite, but that would just make or break the gameplay since counting on the ADC to commit. Prefer ignite and flash for a secure kill and avoid a double kill from the enemy team. Also as my first item I'd go for a goddess tear as it helps reserve more Mana.