For the most part, it was done by telegraph. Most ships of WW1 were coal-powered, and didn't have the range to travel long distances without refuelling somewhere. The places where they stopped to refuel were linked in to the global telegraph network. Messages could simply be sent to where ever the fleet was expected to stop next, to be collected when the fleet arrived. Destroying telegraph stations was expected to be a major feature of cruiser warfare in this period - doing so would paralyse any attempt at pursuit, as the pursuers could not co-ordinate. Indeed, the German cruiser Emden would be sunk after trying to destroy the telegraph station in the Cocos Islands. Radio, or wireless telegraphy, would also be used. This tended to use simple spark-gap transmitters, transmitting in morse code, though more modern ships used the Poulsen Arc system. Spark-gap transmitters were simpler, but more vulnerable to damage and interference.