How to convert the CDT to excel

You just create an excel file that you want to have serve as the underlying template behind the generated document.  Its contents (if any) are fairly optional - but I almost always add a header row with friendly-formatted column names, maybe bolded, with an outline or underline (as in cell borders), and pre-apply filtering.  Anything that displays outside of the inserted cells from the export will retain its formatting from the template you provide. 

You then upload this template to your Appian folder structure, and in the box you've circled, you provide a reference to the uploaded file (i usually suggest creating a constant that points to it, and referencing that in the "base_template" field, since this makes it much less painful to switch to a new template in the future if ever needed).  And if you've created a header row, you simply set your "starting_cell" value as the upper-left-most cell name where the data should start populating (i.e. "A2", etc).