How to use Nikon d7 100 gadget dk-5?

Nikon DK-5 eye mask is used when the camera is exposed for a long time (for example, using door B) to prevent ambient stray light from entering the eyepiece and affecting the imaging effect, and also to prevent debris from scratching the eyepiece when the camera is stored.

When using, first remove the rubber optical cup (shown by the red arrow), and then insert the eyepiece cover into the external card slot of the viewfinder.

The way it's installed.