This is a relatively rough way to try Yuumi mid.. Unfortunately u don't have a lvl 2 powerspyke.. Aswell as you don't have a lvl 3 or 4 powerspyke.. So you're concentrating yourself on farm.. Try to farm as much as possible under turret. But at least try to get some AA's or Q's onto him.

TRY TO FIND OUT WHICH BRUSHES YOUR OPONING LANER IS PLACING WARDS... If your JNG is wanting to gank mid you hop onto him, supporting him. You should fire an aimed Q through the minions which (IF HIT) slows the enemy so your JNG can close the gap. If your Oponent is ranged then he'll likely try to poke your JNG down E in order to heal your JNG. After the Gap is closed you hop off your JNG (Your movementspeed is pretty slow so you take advance of fast JNG or even JNG with a Gap-closer (VI KHA'ZIX LEE SIN // Hecarim // OR (if your JNG is kinda trolly (likely to happen bc ur f'in Attack-Speed-Crit-Yuumi-MID) Pyke JNG)


Once reaching lvl 6 you need to ask your JNG (If pre or not) for a gank..
Since you should farm under turret or relatively far back you're laner could be overextended. Ping (or tell //PREMATE JNG) Which side is warded and let him come from the other side. Start with R to let your JNG close the gap. Hop on your JNG and let him do the first (other to your poke) dmg. Heal him if necessary. Hop off and AA, exhaust or ignite. when he tries to get away directfire a q or hop again onto your JNG and fire an aimed Q. But after all your farm is way more important. Your Overgrowth is going to give you health over time.. So you're literally farming for your life ([weird english with french accent] haha get it?)