What is the first step to sum using source cells in Excel?

To sum using source cells in Excel, the foundational step is to select the cells you wish to sum and then click AutoSum. This process leverages Excel's AutoSum feature that quickly calculates the total of selected adjacent cells without the need to manually type the function or engage in additional navigation.

When you click on the AutoSum button after selecting the desired cells, Excel automatically detects the relevant data range and generates the corresponding SUM function in the destination cell. This streamlines the summation process significantly, particularly when dealing with large datasets where manually inputting cell references can be time-consuming and prone to error.

The other options, while they illustrate different methods of performing summation or using functions in Excel, do not represent the immediate first step in effectively utilizing the AutoSum feature, which is geared towards enhancing efficiency in data analysis and management.