Yuumi is very specific and isnt good as most of supports. Your ideal playstyle is to hit q's(wich can be hard, try to go with your q around minions to hit enemies) and heal as much as you can(this can be tricky, your e doesnt heal but gives mana so try to see wich of your teammates has the least mana and give him it, for your r its very good to catch oponents that are running away from you(if you hit them 3 times they are slowed for good ammount)). Your w isnt good on other champs but with your bf(passive) it heals them for some ammount(only autoattacs(best with chamiopns that hit very fast))