have both open... go to the cell on the destination workbook and press = then click on the cell in the other workbook.
Save the destation workbook
Close the orginal workbook
Close the destination workbook
Reopen the destination workbook and the formula will be there.
Or If the source workbook is saved on C:\sheets\ and called book1
Then the formula to copy cell A1 looks like this
='C:\sheets\[Book1.xlsx]Sheet1'!$a$1