If Excel is not highlighting cells in the formula, enable 'Allow editing directly in cells' and 'Enable fill handle and cell drag-and-drop' in Excel options.

In the subsequent pop-up window, enter the values or cell references you want to use for the function’s formula. You’ll see a brief description of the result you’ll receive.

Clear the contents of cell A1 by selecting the cell and pressing Delete. Then, in the same cell, type the equal (=) sign, and start typing the name you just gave to your new function. When you see ...