1. Click on the cell containing the formula you want to drag down.
2. Click on the cell reference you want to keep constant (in this case, $L$30).
3. Type a dollar sign ($) before the column letter and row number, like this: $L$30.
4. Press Enter to lock the cell reference.
5. Click on the cell containing the formula again.
6. Hover your mouse over the bottom right corner of the cell until you see a small black cross.
7. Click and drag the cross down the column to fill the formula into the cells below.

Now, when you drag the formula down the column, the L30 reference will stay the same while the L1 reference changes for each row. This is because the dollar sign locks the cell reference in place, preventing it from changing when you drag the formula down.