15 or 30 free hours

Hi guys. Anyone super confused about how the 30 free hours childcare works? Anyone not have a steady 16 hours a week? Like sometimes I could have but then not other weeks… so hard to work it out. Is there a deadline which you can apply for the hours and can you change the amount after your child has turned 3? Like if I got more work when they were 3.5 years old could I apply then? How do people spread there 15 hours if they only get 15? Thanks