John,

You can use the 'SDK Script' activity to accomplish this.  A couple of caveats; 

First; the account that workflow is running under _must_ have appropriate Windows permissions to write the CSV file to the target folder.  This is generally not a problem when you are trying to write a file to the local server and workflow is running under the local system account, but it will be a problem if you are trying to write to another server share and workflow is running under a domain account that does not have permissions to the target folder

Second; the routines to write CSV files can get complex if the row data contains delimiter characters (commas in this case).  The code below is about as simple as it can get because it assumes that the row data does not contain any commas.

Finally, in the example below I am 'hard coding' the header row and data row values.  In a real world application you will most likely be storing the document metadata field values as tokens and then referencing them in the script to build the header and/or data row strings.  (If you need help with referencing those tokens in your script then let me know)

Here is the code;

        Protected Overrides Sub Execute()
            
            'Wrap the code in a Try/Catch to catch any errors...
            Try
               'Instantiate a new streamwriter...
                Dim csvWriter As New System.IO.StreamWriter("C:\TestData\Test.csv")
                Dim headerRow As String 'Will hold the header row values
                Dim dataRow As String   'Will hold the data row values
                Dim csvData As New StringBuilder    'We will append all of the data to this string object

                'Here is the header row...
                headerRow = "Column Name 1,Column Name 2,Column Name 3,Column Name 4"
                csvData.AppendLine(headerRow)

                'Here is the first data row...
                dataRow = "Value 1, Value 2, Value 3, Value 4"
                csvData.AppendLine(dataRow)

                'Here is the second data row...
                dataRow = "Value 1, Value 2, Value 3, Value 4"
                csvData.AppendLine(dataRow)

                'Now write the CSV file and close the streamwriter...
                csvWriter.Write(csvData)
                csvWriter.Close

                'Cleanup...
                csvWriter = Nothing
                csvData = Nothing

            Catch ex As Exception
                'Send any error messages to workflow so they can be tracked...
                WorkflowApi.TrackError(ex.message)

            End Try

        End Sub

Let me know if this helps!