First, select the ``Goal Seek...'' tool item from ``Tools'' menu. Specify the output variable cell (``Set Cell'') by typing the cell reference into the entry or by clicking the worksheet cell. If you are searching for the break-even point, for example, you should specify the cell reference of the profit calculation here.
Specify the desired result for the output variable cell into the ``To Value'' cell. In the search for the break-even, specify this to be zero.
Specify the input variable cell (``By changing cell'') by typing the cell reference into the entry or by clicking the worksheet cell. In the search for the break-even, specify the cell reference of the sales here. When you have done this, you may want to press the ``Apply'' button to start the tool.