To apply for a second child certificate, you need a marriage certificate, household registration book, ID card, birth certificate and other documents, and go to the township family planning office to apply for it. To apply online, you need to log in to the official website of the local Health and Family Planning Commission to fill in the information and choose to pick up the certificate by yourself or by express delivery. If there is no online application system, you can apply to the Family Planning Commission of your place of residence or place of residence.