The cylinder is pressurized by the fresh water pump. Simply switch that off (after getting enough for the kettle so you can have tea breaks during the operation), then when you drain the cylinder, no more water will be pumped into it.
On mine, the cylinder sits right at the engine room bottom and the water would drain into the engine room bilge (not the engine bilge under the engine sump, which is separate). It's a bit of a pain to dry that space out and the cylinder sits right in it, so my plan is to dismount the cylinder (held by two brackets) and lift it up enough so it can drain into an oil pan from where I can pump it out. All the hoses are flexible enough to lift it up that much.
When you're done, open all taps (don't forget the transom shower if you have one), switch the pump on and close the taps when they stop sputtering air.
