create SQL Server Mirroring State Change messages

--How to create  SQL Server Mirroring State Change messages

EXEC msdb.dbo.sp_add_category

    @class=N'ALERT',

    @type=N'NONE',

    @name=N'Mirror' ;

EXEC msdb.dbo.sp_add_alert

       @name=N'DB Mirroring: State Changes',

       @category_name=N'Mirror',

       @wmi_namespace=

       N'\\.\root\Microsoft\SqlServer\ServerEvents\instancename',

       @wmi_query=N'SELECT * from DATABASE_MIRRORING_STATE_CHANGE',

       @enabled=1