In the most basic terms, one needs to open a TCP/IP connection to the other, or they both need to connect to some third-party cloud service, then they can exchange messages where one tells the other what to do. Whichever is opening the connection (the client) needs to know the IP address of the other (the server), then open the connection, then send and receive data in some format that you'll have to specify. It's not a trivial undertaking. I can't do it for you, so you'll have to start somewhere, then ask specific questions when you run into obstacles. Emphasis on specific, and on questions; if you can't phrase your post in the form of a question, then it's difficult to answer it.
