Click on the New button.  The Files to Backup dialog box will appear.

 

If you want to specify files to backup, you can do that in two ways. 

 

One way is to click one of the buttons under the word, Select.  These buttons will select or deselect a range of files in the present directory and its subdirectories.  To backup all text files in the Windows directory and its subdirectories, click on the Windows directory in the Folders box and then press the button with the Text icon on it.  If you want to specify other groups of files, press the More button.