A lot depends on how much data you need to bring over. The simplest approach is to have both the workbooks open while setting things up. Then, start typing a formula with the "=", and navigate to the source workbook and select the cell(s) of interest. XL will write the formula for you. Could look something like:
=SUM('[Other Workbook.xls]Sheet2'!A:A)
When you're all done, close the source workbook, and you'll notice XL changes the above formula to have full address
=SUM('C:My Documents[Other Workbook.xls]Sheet2'!A:A)
Now, when you open the Dashboard workbook, all the cells which link to the other workbook will update.