You need to provide placeholders in the document template (field name in curly brackets). It's as simple as that.
If the same field is repeated in other parts of the template we will detect that and fill the document accordingly (with the value provided only once).
Fields could be used inside Sections
Recommended Field Name Rules
Omit special characters and spaces in field name. It will be easier to refer them in Smart Fields
Use Special Filters to modify results