You then have the choice of using Apple's default password or creating your own. Only a ning-nong would use the default password, so I won't even go into that. Click on Change Phrase and type in a phrase of your choosing. You also have the option to have the voiceprint phrase appear on-screen, so it's easier to remember, and also easier to guess. See above, under ning-nong.
Contrary to what you may read elsewhere, the voiceprint you create does not need to be the same as your File Sharing password. In fact, I recommend that it shouldn't be. Why? Because your typed password is probably quite short, a three-word phrase at best. Such a short password will be unreliable as a voiceprint. By the same token, you shouldn't use too long a voiceprint, since you're unlikely to say a long phrase exactly the same way twice. About five to seven words is ideal. (I've used a four-word phrase for this example. However, I should point out that "There is no spoon" proved to be an unreliable voiceprint and I now have a much better, six-word one.)