Multiplayer is not simple, which I guess is why it comes under intermediate and advanced. For C2 I would say that the tutorials are the best way of learning how to use it. Personally I found previous knowledge also helped. It's such a broad topic that a step by step tutorial would not really be of much use. It's not like any other object in C2, where you can just call upon it when you need to, you really need to design your game (or at least the multiplayer aspect) and use it as the core that you are working with.
I can try and answer specific questions to do with the topic, but a whole "I don't understand" is to vague in this large a topic.