Select the range of cells you want to format
Go to Home > Conditional Formatting > New Rule
Choose Use a formula to determine which cells to format
Enter the formula `=MOD(ROW(),2)=0` to highlight every other row
Click Format and choose the fill color or style you want
Click OK to apply the rule
To highlight alternating rows starting with the first row, use `=MOD(ROW(),2)=1` instead
If your data starts on a different row, adjust the formula accordingly, such as `=MOD(ROW()-1,2)=0`