The things I worked on doing is farming, and positioning. Go into a bot game or even normals and just focus on last hitting the minions. It takes a while but soon it'll just be a habit. Keep your auto attacks down to a minimum and practice, practice, practice. After you get the hang of it you'll be able to throw harassment out to the other bot lane while you last hit. With positioning it also takes practice. Be aware of the enemy champions and how fast they'll be able to get to you. A tanky front line makes positioning easier. Just deal damage....to everyone. Your job is just to be dishing out damage so don't worry too much about focusing on just one person, that's how you tunnel vision and that can get you killed because you won't be aware of where the enemies are.
So to recap,
Practice last hitting,
Be in the right place during team fights,
Don't tunnel vision,
Do damage to everyone.
Good luck!