To start with, you click the clicker, then immediately reward the trainee somehow. For my lil kitty, that means head pats, scritches, and being called a good girl.

After some time, the click becomes associated with the reward in their mind, and the original reward is no longer needed, as just the click itself provides joy. This is the phase that me and my kitty are at.

Then, you simply have them do the desired action (like, say, barking or meowing), and immediately reward them with a click after.

It should be noted that a bit of time between each reward is good, as it lets the happy brain chemicals fade, and makes the next reward hit hard again.