The easiest way is to open both workbooks, then type = in the destination workbook and click a cell in the source workbook.
I suggest that you save both workbooks in the same folder. When you reference A1 of Sheet1 in Book1, you may enter this in Book2:
=[Book1.xlsx]Sheet1!A1
If Sheet1!A1 is defined with the name “Basis”, your entry in Book2 becomes shorter, like this:
=[Book1.xlsx]!Basis
