// Creating new workbook
var workBook: TZWorkBook;
...
workBook := TZWorkBook.Create();
try
  workBook.Sheets.Add('My sheet');
  workBook.Sheets[0].ColCount := 10;
  workBook.Sheets[0].RowCount := 10;
  workBook.Sheets[0].CellRef['A', 0].AsString := 'Hello';
  workBook.Sheets[0].RangeRef['A', 0, 'B', 2].Merge();
  workBook.SaveToFile('file.xlsx');
finally
  workBook.Free();
end

// Editing exists workbook
var workBook: TZWorkBook;
...
workBook := TZWorkBook.Create();
try
  workBook.LoadFromFile('file.xlsx');
  workBook.Sheets[0].CellRef['A', 0].AsString := 'Hello';
  workBook.Sheets[0].CellRef['A', 0].FontStyle := [fsBold];
  workBook.SaveToFile('file.xlsx');
finally
  workBook.Free();
end