You submit an "Application for income tax reduction" (main form and children's annex) under "My Elster"(www.elster.de). Authentication is required for this. You can submit the application in person or in writing by post to your competent tax office.
Note
If the application is submitted in person, the child allowance is usually noted immediately in the ELStAM. The employer can retrieve the child allowance with the next change list in the following month.