Locate the three sights on one of the four short side rails located on the long side of the table between the center and corner pockets. Place your yardstick at the center one, and extend it across the table, perpendicular to the side rail.
Find the approximate intersection with the center line of the table (running from the center sight on the head or foot rail to the center sight on the other end). Lightly mark a short line along the yardstick's edge in this spot with the chalk.
Circle around to the end of the table. Line up the yardstick with the center spot so that it is perfectly perpendicular with the end rail (pointing toward the other end of the table). Where the yardstick intersects with your light chalk mark, make a darker, small chalk spot.
Apply a pool table spot to this darker spot. Remove excess chalk markings.
Repeat the process for the other end of the table.