Once you are in the form editor, to add a field:

1. Click 'Add Field'.
2. From the field type selector, choose your desired field type. (Eg. Short Answer.)
3. Configure your field via the dashed light-yellow box. (Eg. Label, Mark as mandatory etc.)
4. Click 'Done' toward the bottom of the light-yellow box.
   

Great! You've added the field!

After you've finished adding fields, don't forget to Save the form (Ctrl + S).