To put it quickly, in the admin console, create the field by going to Data\Custom Fields and selecting the module you wnat to create the field for. When finished, go to Presentation\Forms, select the node, then the appropriate form to add the field to. The form and an attributes box will appear and you simply drag and drop the field into place.
That is the very short answer, but for more details, I highly recommend the help files of the admin consol. There's a whole section in the contents for Adding custom fields.