After creating a database (and uploading it), you need to create a MySQL user and add it to the database. To do this, go to cPanel/MySQL and under the User section, enter the username and password. Once the user is created, add it to the database by clicking the “Add User to Db” button. Both usernames and database names have the format userdecpanel_databasename and userdecpanel_mysqlusername.
Video tutorial for MySQL database configuration