Clear the table top so it is easy to take measurements of the table.
Use the tape measure to determine the length and width of the table. Have a friend hold one side of the tape measure as your run it across the table.
Write down the measurements for the length and the width on a notepad.
Use the tape measure to determine the length of the desired drop. Hold the tape measure at the edge of the table and let it run towards the floor, stopping at the desired length. Write down the measurement.
Multiple the measurement for the drop by two. Then add that result to the length and width measurements. The final result will be the proper measurement for the tablecloth.