Ok, just a real quick rundown. The instance of the game that is the server/host should be the only instance that decides where objects are, the state they are in and the actions they are taking. It should then send out a packet of information to all connected clients updating them with all the information they need to display properly.
All client instances should only collect user input and send it to the server, receive updates from the server and draw the clients viewport from the updated information.
How do you do this? Well, there are LOTs of different ways to handle it. Some games have a separate program that just runs as a server with all the server code and a client program with just the send/receive/update code. Others use a variable which defines if the current instance is the client or the server and runs the necessary code accordingly. Construct has an IsHost expression that can be used for this.
Unfortunately, you are going to have to evaluate what will work best for your specific project.