I created all databases. I want to know how to assign privileges to an existent user.
I read this:
One final step is to make sure you have assigned a username and password to be associated with each database. This could be the same as your username and password for the original WordPress database. It’s up to you, just make sure that there IS a username and password associated with each one and note what it is. You’ll need this information for the next step.
But I do not know the username and password to access my databases. I just created them by using the code generated by https://premium.wpmudev.org/db-tools/db_sql.php