Each country within EBBA2 scope has its own national coordinator(s). If you are interested, please, contact your national coordinator, who will provide you with thorough information on methodology and data submission. See the contacts to the national coordinators.
You could also explore whether your country already has an existing on-line portal designed at entering bird data in Atlas format (specified 50×50 squares and Atlas breeding codes). As a starting point, you can check the EuroBird Portal (on-line platform with partners from 21 European countries), and look at quick overview of their partners.