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Well, assuming that you have a cloaking device installed on board, then you activate it by using a computer console (probably the tactical, helm, or engineering console on most vessels) to bring it online. Also, assuming you're on a 24th century starship (not, say, Enterprise NX-01), then saying "Computer, active cloaking device" would probably work, too, as long as the computer is fully functional.

Of course, most Federation vessels do not have cloaking devices on board, so if you're on board one of them then this is somewhat of a moot point. :-)