I dissent from both the answers. Don't create a reference at all, but use late binding:
Dim objExcelApp As Object
Dim wb As Object
Sub Initialize()
Set objExcelApp = CreateObject("Excel.Application")
End Sub
Sub ProcessDataWorkbook()
Set wb = objExcelApp.Workbooks.Open("path to my workbook")
Dim ws As Object
Set ws = wb.Sheets(1)
ws.Cells(1, 1).Value = "Hello"
ws.Cells(1, 2).Value = "World"
'Close the workbook
wb.Close
Set wb = Nothing
End Sub
You will note that the only difference in the code above is that the variables are all declared as objects and you instantiate the Excel instance with CreateObject().
This code will run no matter what version of Excel is installed, while using a reference can easily cause your code to break if there's a different version of Excel installed, or if it's installed in a different location.
Also, the error handling could be added to the code above so that if the initial instantiation of the Excel instance fails (say, because Excel is not installed or not properly registered), your code can continue. With a reference set, your whole Access application will fail if Excel is not installed.