How to add the certificate, and any others above it in the chain, to the database is explained.
Before you receive your signed personal certificate into the database, it is important that you add the signing CA certificate. If the CA certificate that you want to add is in a certificate chain, you must also add all the certificates that are above it in the chain. You must add the certificates in strictly descending order, starting from the root, followed by the CA certificate immediately below it in the chain, and so on. Certificate chains are explained in How certificate chains work.
To add all appropriate CA certificates, follow the procedure in Add trusted certificates to keyring.db.