Well, first you load your vehicle as it would be normally.
Then, you weigh the vehicle.
Then, using the tire loading/inflation charts provided by the tire manufacturer (usually available at the manufacturer's website), you inflate the tires to match the load, making sure that the tires on any one axle are inflated equally.
Or, you just go by the tire size/weight chart on the door jamb of your vehicle. the tire inflation pressures listed there are the correct pressures for THAT vehicle when loaded to the maximum rated weight.
Or, you can try the "chalk test". Make a chalk mark across the tread of the tire. Drive a few feet. Is the chalk mark evenly worn? The pressure is just right for the load. Is the chalk mark worn more on the edges? The tire is under inflated. Is the chalk mark worn more in the center of the tread? The tire is over inflated!
The pressure on the tire side wall is for the MAXIMUM load the tire can safely carry. Are you REALLY loaded that heavy?