To install the new focusing screen hold it by the outer tab with tweezers, and lower it into the housing, opposing-end of the focusing screen first. Then gently set the tab-end into the housing.

Now grab the latch of the housing with your tweezers and raise the housing up into its original position, above the mirror, applying gentle pressure until it clips into place.

Finally, re-attach the lens, insert the battery, turn the camera on and test the focusing ability of the camera.