Hello,
Please review this document that explains how to add a service account to your Cloud SQL instance so it can be used to access it. Connecting to your instance will depend on where you are connecting from. You are able to review the following document about the Cloud SQL auth proxy which will give you more understanding on how to properly connect from different environments.
If you are not able to connect to your instance after following this documentation, I would recommend you to contact GCP support where they will work with you to make sure you are successfully able to connect to your instance.