This should get you started...  

SELECT
  s.[bp_instance_id],
  s.[submission_id],
  i.[bp_name],
  d.[attribute_id],
  d.[value]
FROM [LFForms].[dbo].[cf_submissions] AS s
LEFT JOIN [LFForms].[dbo].[cf_bp_data] AS d ON d.[submission_id] = s.[submission_id]
LEFT JOIN [LFForms].[dbo].[cf_bp_main_instances] AS i ON i.[bp_instance_id] = s.[bp_instance_id]
WHERE i.[bp_instance_id] = 123456

Instead of 123456, list the instance ID of one of your forms.  Run this to see all of the field data submitted across all of the submissions in that process.  Once there, you can determine what attribute_id is used for the desired field by reviewing the data values submitted to the value field.

Once you have that, replace the last line of the query with one that searches that attribute ID and the name of the business process, like this: 

WHERE d.[attribute_id] = 1234 AND i.[bp_name] = 'Process Name'

You should now have a query that lists every submission of that process with what was listed in the desired field.

Note that forms that have multiple tasks that include multiple submissions that could modify that particular field - if that is the case, you'll likely want to tweak your query in order to retrieve the value populated by the last submission_id within each bp_instance_id.

It'll take a few more leaps to make this in to a usable report, but hopefully this is a good jumping-off point.