How to mid?

I'd like to know some more about mid lane, I know that the mid laner should roam a bit after lvl 6 and all, but I'm so bad at it. I play Akali, Yasuo and Ahri. When I play as Ahri I usually never roam but I know I **have** to roam when plaing as Akali or Yasuo, since I don't roam I come behind most of the time and we lose. I know how to play Akali but I'm still not good at it, but when it comes to Yasuo I'm terrible, early game goes good, I try to roam, my lane is lost in no time. So how do I play better in mid lane?
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