Use quas as your advantage since you will have quas to help regenerate your hp , do not be scared to harrass your enemy everytime you can and do not be scared to creep hitting you.
This is very important because most other mid heroes has more damage than Invoker but has Quas to regenerate!
Early game: Cold snap -> EMP/ Tornado / vice versa
and in early phase of the game whenever you can hit 2-3 hits on him just do it
For escaping or enganging
remember that with the newest update ghost walk give movement speed which you can use it to your advantage
Teamfight: Tornado -> EMP -> Meteor -> Deafening blast (you must have very quick hand) -> Ice wall ( only use meteor when you have agh if not just skip it to deafening blast)
For solo kill Tornado -> EMP -> Coldsnap
Remember it has a very far range
ColdSnap -> EMP
or
Tornado -> EMP
Carry Invoker
Orchid combo: Orchid -> ColdSnap -> EMP -> Tornado
or vice versa : Tornado -> EMP -> ColdSnap