Set the speaker over the bottom earpiece, making sure you align the screw channels as well as the air channel.

It should sit quite tight to the bottom with only a small gap left. You will ear a "click" when it is right.

Put back the 3 screws and it should sit quite nicely.

Note: At that point, you should be able to test it everything works and the reparation was successful. Give it a check before you continue.