Once you've done this four times, you'll have the option to Try it. It's a good idea to test it before you leave Multiple Users, because it's a little tricky to get back in if your login doesn't work. If the test fails three times, you'll be prompted to re-record your voiceprint, possibly with a different phrase.
If the voiceprint is OK, you'll get a jolly little message telling you you're ready to use your voiceprint next time you log in. And you're done.