You get the remaining amount of the child tax credit by filing a 2021 tax return. For 2021 only (unless Congress changes it ) you do not have to have earned income to get the CTC. So prepare a 2021 tax return claiming your child and it will be given to you as a tax refund.
You should not attempt to file 2021 tax forms until the end of January or early February...the appropriate forms may not be ready until way into January
You may be a stay-at-home mom, but if you are married and filing jointly.... do not file those 2021 tax forms until you have received any and all income forms that you or your spouse may be getting for 2021, sometime in late Jan or early Feb. Every W-2 (spouse?), or 1099-??? type of form you receive, for things like dividends, interest, or investments, must be reported in your tax filing. IF you file without including those forms, it only leads to amendment nightmares later.