Loosen the kingpin (the large bolt in the center of the truck) until the trucks are loose enough to rock back and forth under decent pressure.
Stand on the skateboard and rock your weight as if you were turning left and right. Continue rocking until you notice the bushings have softened, and the trucks themselves feel looser.
Flip the board back over, and adjust the kingpins to your desired tightness. Test the board out until you find the perfect setting.