You will wait 2 days and one night. On the second night right at midnight (12:00am) it should get up and look at the baby for 3 minutes. Then open its mouth 2 or 3 times and it will fly up in the air. Then at whatver time the parent wakes up, thats when the new genoration starts.
Or, after the first night, on the second day, you can set the time to 11:59pm and wait one minute. Sometimes it doesn't work the first time, if it doesn't, wait about an hour and do it again. Once the parent flies away, set the time to (whatever time the parent woke up at) and you'll start the new genoration!
Hope I helped!
~*Tamagotchi*~