There are 3 plastic membranes, the key presses are created by the top layer points pressing against the bottom points.

in between the top and bottom layers is a clear piece of plastic membrane.

The key to repairing any broken keys is to find them, then follow the green line for each broken key back to the next key that works.

Once you find a key that works on the same green line as your broken one, you can...

Draw a line with your conductive pen to connect the broken and working key.

Remember, only on the same green line. If you cross lines your keys will not work correctly.

You can see the dark lines on my pictures connecting the broken keys. In some cases, i've gently scratched away the green covering to expose the wire underneath, and connected my keys to that instead of drawing a long messy line.

Often the key doesn't work without drawing lines on the top and bottom PCB membrane. Both sides will use slightly different routes, so remember to follow your green lines carefully.