Step 1: Orient the hinge
The plastic hinge mechanism must be oriented as shown for it to fit onto the metal keyboard hooks.
Step 2: Hook the hinge onto the base
Hook the inner bottom edge of the hinge under the bottom metal hook on the base of the keyboard.
Step 3: Secure the hinge into place
Snap the top of the hinge into place, one side at a time. In this example, the top right side is being secured first. The top right and left corners of the plastic hinge should be hooked under the metal hook 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 it's axis.
Step 4: Press the key into place
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.