It’s frustrating to toggle back and forth through Excel windows to find the various components of a formula. One way to make it easier is to look at multiple worksheets or workbooks at the same time.
Open the workbooks you need. Click the View tab, and choose Arrange All and either vertical or horizontal. To use another work sheet in the same workbook, first click the View tab and click New Window, then Arrange All. Now just type an = sign to begin your formula, and click the field on the other work sheet or workbook to add it to the formula.