you dont implement a interface by making a variable. you gotta go to the bp which needs to implement the interface and on the details panel of that bp(if you dont see the details panel or cant find what im gonna talk about click on class defaults on top). and then there will be a list of implemented interface. you can add your own interface in that list.
Interfaces will also be inherited if you inherit a BP.
here is a wiki page if you didnt understand what i said: Implementing Blueprint Interfaces | Unreal Engine Documentation
P.S: the above wiki page uses level blueprint as a example, u can use any other bp aswell.