Here is a direct link to the portal that let’s you sign up to adjust your child tax credit — and, in your case, add a newborn. That means you will start getting a monthly advance, probably in November if you do it now. But no matter what month your child was born in 2021, you get the ENTIRE tax credit for the year. You will apply for it when you file your 2021 tax return next April (minus any checks you start getting now). And you may even apply for the Earned Income Tax Credit if you qualify — and receive more for this additional child.