First, the diff is placed in the case and the case halves are bolted together. The torque to rotate the diff is then measured with an old stub axle connected to a torque wrench. If the torque to turn the diff is too little, a thicker shim is used to increase the preload. If it is too much, a thinner shim is used to decrease the preload.
Fortunately for us, the OS Giken diff had the exact same dimensions as the stock diff and the preload was perfect as is. Unfortunately, that gives us no pictures to show how preload is set exactly!