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Well, for proprietary protocols that you don't have a specification document for there are two options:

1. reverse engineer the protocol by playing with the client to see what kind of data is sent as an reaction to whatever you do, e.g. "if I move in this direction, data looks like this, if I move in another direction, those bytes are different". It's basically a differential analysis
2. reverse engineer the game binaries to find the part that generates the network data. This is extremely hard to do unless you are very skilled in reverse engineering binaries.