Now you have to put all the components back in the new case, to do that please refer to the following steps

Replace the proximity sensor and the blue plastic piece back in the new case, first remove any protective film that may be present

Now peel off any eventual protective film, one may be placed on a copper heater sink

Now you have to apply a new thermal paste on the heat sink as in picture

Then you can place back the motherboard and gently push it in place with your fingers until you find it very firm in its position