This guide is for hinge type K27. This hinge type is found in the Apple Macbook Pro Unibody.
Some colors in the pictures may differ for your model. That is fine, what matters most is the shape of the hinge piece.
The plastic hinge mechanism must be oriented as shown for it to fit onto the metal keyboard hooks.
Hook the inner right edge of the hinge over the right metal hook on the base of the keyboard. Here we are using pliers but this can be done without tools.
Tuck the left side of the hinge into place, one corner at a time. In this example, the bottom left corner has been secured first. The bottom and top left tabs of the plastic hinge should be hooked under the metal loop protruding from the base of the keyboard on each side. You can now check that the hinge works by flexing it up and down; it should pivot easily at its axis.
If your keyboard is missing the rubber spring piece, place it in the center of the hinge assembly. 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.