The two plastic hinge mechanisms must be oriented as shown to fit into the base of the keyboard
Hook the plastic tab on one side of a hinge under the socket on the base of the keyboard. In this example, the bottom of the left hinge is being put into place first.
Squeezing the other side of the hinge, tuck the remaining plastic tab into the base of the keyboard. You will probably need a tool to do this, like a pair of needle-nose pliers. Repeat steps two and three for the second hinge on the other side. To fit the key on easily, make sure both hinges are moved out from the center, all the way back into their sockets.
If your keyboard is missing the rubber spring piece, place it in the center of the hinge. It will be held in place by the surrounding key and hinge.
Align the key on top of its place on the keyboard, and press down evenly. All four corners of the key should click into place.