When and how do I add the cue?

This may help - I like Melissa's explanation:
[12:50:34 AM] Desperado Pets: When and how do I add the cue?

In clicker training, we add the cue after the behavior is shaped and strong, not while the dog is learning it. Why?

For two reasons: First, when the pup is learning the behavior, we want him to concentrate on the behavior. At that point, the cue is meaningless to him anyway—just another bit of "noise" to sort through. In the beginning, make learning easier on your dog by minimizing distractions, including meaningless cue words. Second, we want the cue to be associated with the final, perfect form of the behavior. If you add the cue in the beginning, you run the risk of having an unfinished version of the behavior crop up with you least want it to—like during the stress of competition—even though you continued to shape a more precise behavior.

First get the behavior you want in the form you want. One of the myths about clicker training is that we go around with a myriad of uncued behaviors for ages and ages. This is just that—a myth. Add the cue as soon as the dog is actively offering the behavior you want. For a simple behavior, that could happen on the first day!

More complex behaviors may take more time to shape. If the behavior is extremely complex—a behavior chain, for example—you can add cues to the individual parts of the chain, and then add a cue for the entire chain when it's complete. Or, if the behavior is a single but very elaborate behavior, you can use temporary cues as you shape it, replacing them with a permanent cue when you've achieved its final form.

Once you have the behavior you want, practice it until the dog is actively offering exactly that behavior—that perfect behavior—80% of the time, then add the cue. Remember, we're just looking for the behavior itself at this point. You'll train elements such as distance, duration, generalization to multiple locations, and proof against distractions after the behavior is on cue.