Once you're confident you've sorted all the broken keys, bring back your white keyholder... Gently, check all the little rubbery nodules are in place in the keys.

Carefully bring the rest of the keyboard to the top of the white keyholder and fold the membrane over the top of the keys.

Carefully place the metal holder on top of the membrane. Checking that you haven't dislodged any of the rubber nodules.