There are two spots on most pool tables, one at the "foot" of the table, where the balls are racked at the beginning of the game, and one at the head of the table. If you have a new table or have just replaced the felt on your old table, you'll probably need to apply new spots. Since tables can be seven feet, eight feet or nine feet long, it is better to use the "sights"--usually diamonds--along the rails to judge where the spots should go.
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.